Making cars different would allow multiple strategies and multiple winners throughout the season. Right now, in real F1, the Red Bull is very quick on high-downforce circuits, the Mercedes on high-speed circuits and Ferrari a pretty all-round car. The Lotus is very good on tyre wear.
In iGP, such a variety would certainly add more dimension to the game if you could choose focus areas for design.
Let's say every year you got 20 points to spend, which you can put in the following area's:
top-speed
downforce
tyre wear
fuel consumption
You could put five in all of these area's, or go 9-9-1-1, for a very fast car that has to stop more. Or 2-2-8-8, for a car that is more likely to race on 1 or 2 stop strategies.
There could also be chief designers who have perks in either one of these fields, meaning your car would be designed around this. It would spice up design, as now it's just meaningless percentages which reset every year, resulting your car is either fast, or slow, the whole season, with everybody using more or less the same strategy.
Inspiration: Dragon Age leveling system http://i300.photobucket.com/albums/nn6/jyenicolson/levelup.jpg