P&G treats problems that some cars do not suffer in the first place. Specifically, excessive engine revolutions, and cruising with the throttle mostly closed. If you can treat those problems without P&G, then your engine is near peak BSFC even during steady state cruising.
Ford was an early adopter of throttle-by-wire, and I'm sure it's included on the new Mustang. The car no longer has a "throttle" pedal, it has a "driver intent" pedal. When the driver requests little power, the ECU is free to upshift and open the throttle wide for better efficiency like a hypermiler would do. Combined with the new Mustang's six-speed automatic transmission that is geared lower than a 5 speed Insight, the variable valve timing, and the 5W20 and low-friction engine internals Ford has been using for the past decade, and you have P&G-like efficiency without P&G.
I have tested the Insight and found little benefit to P&G over lean burn:
http://ecomodder.com/forum/showthrea...p-g-13206.html
Conventional engines are closing the mpg gap during light-load cruising, but hybrids with efficient energy storage and retrieval can still deliver much better fuel economy around town.