To me it's not a question of CPU power, but form factor and user experience. Hybrids have for me two big drawbacks --- and it's really subjective:
- I like small tablets and larger laptops (13.3 min, 14 prefered). So an hybrid is either a too big tablet or a too little PC, often both;
- hybrids are top-heavy, and I don't like having an hinge. I really want a LAP-top, and a ultrabook is perfect for this. An hybrid, not so much (I've heard about "lapability", not convinced ;).
Both are intrinsic trade-offs of hybrids. So I stick to small tablets + 14" ultrabook, and just don't bother with hybrids. But to each its own.
Ultrabooks/Subnotebooks are already a compromise. We've made the transition from massive 30-40W CPUs to 15W-CPUs for mainstream notebook over the last 4 years, where net performance for notebooks stagnated at best (and regressed during the first year or two). Going from there to whatever hardware the Surface Book uses won't a big leap any more.