Until we see more tests done on the demo hardware and get more information about the 80% of the die area allegedly not concerned with implementing speculative threading (or whatever it is you care to call it), it will be difficult to make any accurate comments regarding its efficiency; ability to scale to higher clockspeeds; or really anything else of substance.
One thing that seems interesting, and a bit odd, is that the provided JPEG/color compression demo that they have released would seem to indicate that there is more than just speculative threading providing an IPC boost to the prototype VISC CPU. Correct me if I am wrong, but the demo shows that two "actual" VISC cores operating as one virtual core @ real clockspeeds of 350 mhz can beat a 1 ghz Haswell core in the same task. If the speculative threading feature is 100% efficient and makes the two physical cores function as a 700 mhz virtual core, you still have a 700 mhz virtual core beating a 1 ghz Haswell core.
Or, to put it a different way, the test seems to indicate that a single "actual" VISC core @ 350mhz would beat a single 500 mhz Haswell core in JPEG/color compression.
That makes one heck of a statement with or without speculative threading. Can the VISC cores manage such a feat in other computational tasks?