I would expect that that was the simplest manufacturing conversion. The MG1 maximum RPM would be the biggest change but that may have been a technical motor windings issue that they solved. I expect that the top speed and acceleration are de-rated. The Prius was such a successful design that stretching it a little was feasible. The Volt has a more properly sized ICE and Traction motor for a PHEV. The X-Prize 100 mpg. winner that weighed 800 lbs. shows the difficulty in achieving much over 60 mpg. in a real world 5 seat, air-conditioned turnpike cruiser. It's interesting how more cars are approaching the Prius body design. My early FFH tests showed an almost linear inverse relationship of MPG. to MPH. yielding 64 mpg. at 30 mph. That would extrapolate to over 70 mpg. at 20 mph. I think you were involved in the 2008 FFH 81 mpg demo that I think averaged about 21 mph. so you guys did some additional stuff, I guess. Getting car weights down below 2000 lbs. is going to get expensive with carbon fiber and electronic shocks, etc. The HVAC is the big killer over 50 mpg. Pumping heat is expensive.