I bought the Syborg boot primarily for racing (newbie racing that is) and long light tours, but I have also taken them out for a day in the deep powder with my wide boards, which are La Sportiva Hi5's. the Hi5 is 105 underfoot and reasonably stiff (and heavy by today's standards). My conclusion is that the boot's ability to drive a big ski is very conditions-dependent. The day I took them out, the conditions were sublime: I did 4500 ft of perfect 12-18" of powder and skied them fast on mostly mellow terrain (low 30's). The boot handled the ski amazingly well - I had no gripes whatsoever. I could turn the ski fast through the trees and also completely let it rip in the open terrain.
That said, those were pretty much the easiest conditions ever. I've also had the boot out in less than ideal (and sometimes very hateful) conditions, and even on light, skinny skis, I've wished for more support and less flex. As soon as the conditions get variable in the slightest, I notice an immediate decline in control relative to other touring boots I've skied in the past (like my TLT5P's).
So, if you tried to ski this boot hard on a wide, heavy plank like a Hi5 in steep, variable, or perhaps less than ideal conditions, I think it will leave something to be desired. In contrast, my TLT5P handles this ski in all of those conditions sufficiently well (with the tongue insert in). Caveat: I haven't yet tried the Syborg's power strap, so maybe that would improve things.
Finally, I'll say that I really love the Syborg, and I think it does exactly what it's designed to do. It's my first race boot, so I can't compare it against other race boots, but it is extremely comfortable for my foot, has excellent articulation and is stiff enough to get the job done. It has completely replaced my TLT5P for long, light touring, and I'll even continue to use it with my big skis when the conditions are just right.