In seeing the direction this thread is going I feel I have to add my thoughts as well...
NYChevoo:
First of all no one who has experience ever said modifying cars to achieve much higher HP than stock was ever cheap, or good "bang for the buck". Its not a cheap hobby nor does it really make any financial sense. But for any enthusiast, the motivations for doing these things lie elsewhere.
You say "...If some of you are interested in turning these little cars into speed demons then why not skip the whole idea and buy a GTI or a Mazda 6 or something like that..."
The whole reason (for me at least) is to have something different and unique, that nobody else has. Something that you have worked hard on and can honestly say that you understand how everything works, and then improve upon it. Anyone can go out and buy a GTI with 200hp (which isn't very much), but they haven't learned or accomplished anything except to open up their wallet and fork over the cash. I do understand that for most people, this is the route they would choose - because its easier. And where does that leave you - the exact same as every other one of the million GTIs out there.
Nemasys:
I know it can be a difficult battle with cars such as these (Aveo, Optra) because of the limited aftermarket support and need for custom parts. But it is possible to have a reliable 2.0 turbo running and daily driven with more power than a GTI or MazdaSpeed
The easiest econo-car of course to make fast now is any Honda with a B series swap. Millions of aftermarket parts available in north america + lots of R&D has already been done = easy 500hp+
I've actually considered doing a Honda B swap into my Optra, but I think that would have to wait until the car is not needed as a daily driver.