But what you're describing is the way that the prius and other popular models do it. What I was trying to say, is that instead of using a smaller motor, you could use the battery power in addition to the regular gas motor when you take off. In the prius, the gas motor doesn't kick in until you have started moving, but what if you ran both the gas and off the battery power at the same time? Then you'd have more overall horespower.
They were talking about doing this on at least one vehicle (probably a chevy or ford suv) and it wouldn't get better mileage, but just have more power.
It is much easier to suggest solutions when you know nothing about the problem.