I can't help you with outright efficiency of the Insight, but I can give you some pointers for engine efficiency in the Prius. Most petrol engine cars use the Otto cycle, where the compression stroke is the same length as the expansion stroke, and efficiency is therefore limited to ~25-30%. However, a more efficient cycle, the Atkinson cycle, has been mimicked in the Prius by simply closing the inlet valves a little late on compression. This means that the Prius engine effectively has a compression ratio of 9.5:1, but an expansion ratio of 13.5:1. The result is a whopping peak thermal efficiency of 36%. FYI, the highest peak thermal efficiency of any engine on the road is the 1.2litre TDi engine in the
Lupo and A2 3L, with peak thermal efficiency of ~45% (diesel engines are inherently more efficient, but this one has been souped up even more).
But, that's not really where hybrids score the most points on efficiency. By having a smaller engine in the Insight, or by using the planetary gear system in the Prius, it means that the engine can run at a wider throttle opening for more of the time (or even full throttle for almost all of the time in the Prius) than a normal car can. Engine efficiency plummets at low throttle openings, and by working the engine harder, you get better efficiency. You can see what I mean in this presentation:
http://john1701a.com/prius/presentations/prius_ths-presentation_07.htm Regenerative braking also restores a small fraction of the energy lost from braking in todays mild-hybrids.
In terms of overall vehicle efficiency (not just engine efficiency) there are a million other variables to think about. The gearbox will lose up to 10% of engine power. The size and type of tyres, the aerodynamics and weight of the vehicle all impact efficiency, so there is almost no way of coming up with an "outright" vehicle efficiency for any vehicle.
