ORIGINAL: Jeremiah 29:11
The Toyota engine never did perform that well in the Toyota car it was in but because of the Lotus relationship with Toyota, the borrowed the engine and it did much better in the lighter Lotus.
You are correct in that is why all Lotus cars excel is because of the design philosophy which is a better power/weight ratio than any other car is the key to performance.
I'd think part of the reason why they did that was for cost reasons. That Toyota engine would be very inexpensive to use and would not need a very expensive transmission to back it up in the category of strength, so they would be able to offer a car that is outstanding in cornering and has acceptable acceleration for its class for the affordable price of $45K. Although you could probably drop in either a EVO or STi engine and get the car to move faster by the fact that it would offer 300hp to offset the weight increase, the cornering might suffer and the price would definitely have to increase as I'm sure those cost more and the transmission would definitely have to be strong enough for those engines.