You can never improve the lack of engineering that didn't go into something like a civic or mustang by throwing a lot of money into it to make it fast. Cars are totally stripped and almost completely reengineered when they're going to be used in BTCC or something like that, they just aren't a suitable place to start for a serious performance effort. Add up the money that it takes to do that within the confines of the rules, and you could have bought an exotic, or two or three. An exotic starts from the right DNA, and is designed around a single purpose from the ground up, often with no regard to cost or practicality. If that's what you're after, there is no replacement for an exotic.. There are exotics that are just really expensive for the sake of being really expensive (Stryker, etc), and then there are the true engineering masterpieces, the Ferraris, Lambos, Koenigseggs, and Paganis of the world. Money doesn't make a brilliant car, a team of brilliant designers does. It just takes money to fully realize the potential of a clean sheet departure from standard design.
If I had the choice I would get a Zonda, because I believe it's one of the purest exotics out there, built in small quanities to an obsessive standard of design and function. Oh yeah, did I mention I'm obsessed with composites and idolize Horacio Pagani?
Someday I may get a chance to become part of a team that designs stuff like this, and will be far more satisfied with that than I ever could be if I restored or modified something else.

So I'm all about exotics. Anything can be made fast, but it's impossible to correct something that starts out fundamentally flawed.