I will go on record as saying I do prefer some flex under foot. Here is what I mean. Years ago, I was riding a 179 CM Morrow Matt Goodwill board with direct mounted Sno Pro toe and heel pieces. This was back when many boards required drilling, like skis, to mount. The next season I got the new Matt Goodwill board from Morrow, but now it had inserts, and, a dedicated Morrow only insert pattern. So I had to try Morrow’s traditional strap bindings. The Morrow bindings had an aluminum baseplate likely 6061, bent into a U shape to cradle the boot. Of course the form of the aluminum baseplate made it very, very rigid. I hated it. It felt like the board had a dead, flat spot in its flex, gone was the great ride of the previous years board. Of course my direct mounted toe and heel pieces did not inhibit the designed flex pattern of the board at all.
After that, when I switched away from hard boots and rode soft boots for a number of years, I preferred Burton’s carbon loaded baseplates. They were fairly stiff and responsive, but not as stiff as a bent aluminum baseplate, and did not give the dead ride feeling that all the aluminum bindings had.
When I started splitting, I realized pretty quickly that I would be going back to hard boots, it just made so much sense, and the new AT boots that were coming out were really light and relatively soft. Also, the Spark binding, even for all its considerable innovation, also resulted in a bit of that dead feeling which I remembered from aluminum strap bindings on solid boards. The Voile plate, by virtue of its shape, is pretty rigid as well, but it is fairly narrow, and the pucks allow for some give (until they break).
Now with the Phantoms I feel like I have it all. The board halves are held in plane, the toe to heel response is really, really stiff and good, and the baseplate of the binding, being a flat plate of 7075, has some flex and give (especially medially and laterally) and allows the board to flex relatively naturally. I would not want the baseplate to be absolutely rigid, I like it to flex some along with the board (lengthwise), but toe to heel, I want response, and this is exactly what the Phantom bindings are designed to do.
One other note. In discussion with Klem (Venture owner/designer), he noted that sometimes really stiff metal bindings will damage boards at their edges, where they point load the top sheet. Since the Phantom baseplate by design rests directly on the topsheet of the board, some flex is engineered in, as well as the corners having some radius to them, as to not overly stress the board and cause premature failure.

Vapor: Pretty much agreed. Phantom bindings do not allow more than 30 degree angles, their partially oval shape compensates somewhat for the property which you are describing, but these bindings were never meant to be used at angles over 30 degrees, and indeed do not allow for angles over 30 degrees. For sure, Phantom Bindings are designed to suit the riding styles of 99.9% of backcountry freeriders who ride at stance angles of <31degrees.
Note that with a splitboard, those who prefer high angle stances, would be better off with a totally different design than that offered by Voile style pucks or Phantoms, as a board interface system at such large angles does not distribute forces across the board very evenly: looks like a potential oppurtunity for someone to develop a binding for the ten or twenty riders riding higher angles in the backcountry, with an interface which crosses the board at right angles, and then a binding above which allows for the stance angle. Of course, this would be heavier though.
I ride at 24-27 F and 3-7 degrees (positive not duck) R. No problems or dead feel at these angles. Also note that this season Keffler made two versions, the very limited production lighter, softer flexing and semi-experimental Pho, and the slightly stiffer proven Alpha model. I suspect he will merge these together for the 14/15 season but am not sure exactly what will happen.
I am not sure how applicable it is to talk about solid board carving bindings here, as these work in an entirely different fashion to what goes on with a splitboard (angles always over 45 degrees, high off of the board surface, much stiffer boots, very narrow boards, binding mounted from the center with damping at the edges, often mounted on a separate plate system, very little or no concern for weight, etc…)
In any case, all good discussion.

Not sure? I’ll have to strap in and lean… my guess would be 26-30deg. I had the top strap lengthened and put in some eliminator tongues to provide more room to lean as well as a softer cushy feel to lean into. Besides filing the lock plate as far as i was comfortable with (maybe 1 cm). I’m also moding the plastic spine/highback by cutting both sides back about 2-3 cm to give more side to side flex. Phantoms on the way..(just add snow!)

I’ve been riding these boots this season with dynafit toes and Phantom bindings. I’ve done a few mod’s to mine which I will eventually post final picks of. My mod’s; trimmed back the vertabrae arms both medial and lateral sides, extended exsisting strap by sewing on supplied extra strap, filed down lean bars (3cm or so) and bolted on supplied spoilers. I was thinking of zipties to hold the strap in place, this should work?
My only issue at this point is the hard stop feeling when leaning back (this is mitigated by the spoilers). I to road the voiles last year and can attest to the fact that Phantoms are a leaps and bounds ahead for ride feel and board mending. The tour mode is also light years away from touring with my old sparks. This is now my dedicated mode of travel for the future. I would like to design a twin tipped ski/board for the future to take full advantage of the ski boots.

Late response to Stew, but I run powerstraps (or Scarpa’s version of them) on my F1s for the upper cuff closer. To keep them from sliding down I just cut a slot in the overlapping portion of the cuff and passed the one end of the powerstrap through from behind. Not a fancy by any means but why add when you can simply take away!

Anyone tried out the new Sideral 2.0? It seems that this year’s model has a few changes that fix Stew’s (@jrsplitr) issues.

Anyone sporting crampons on these boots? Ordered Black Diamonds Sabretooth, but these didnt fit at all.

@beard-of-power I seem to remember Wildsnow saying Tech Crampons were cool if you never Fernch-stepped and knife-hard surface snow was never an issue where you go. I.e. it’s a nice quiver crampon (especially where grams count) but is unfit for general use.

@gaston I think La Sportiva redesigned the radius of the rocker in Sideral 2.0 to fit more crampons.
This guy says Grivel’s Haute Route fits another La Sportiva boot (with the same rocker-crampon-interference). Maybe it will work with Siderals?
Lastly, I know it goes against the whole hardboot ethos, but have you tried regular strap (universal) crampons?

A buddy of mine says he got it to work by swapping the toe bail and modifying the boot.