The stats do increase as you add the upgrades.
Also as regards buying the new cars, the most popular Rally cars are often the 350Z and Evo X, with plenty of new WRXs as well. That said the new WRX is actually quite slow due to understeer whereas the Evo X is very stable and easy to drive whilst not being to slow. The Z I'm not to familiar with but people seem to make it fast in ranked games at least. Other then that, the Escort is fast, fun, but difficult for some.
Trailblazer on the other hand, the Solstice is what I'd recommend as it is probably the best outside of the boring Dallenbach.