Well it most certainly will affect performance. I'm not sure how that adapter works, but I can assure you you'll probably have no electronic control of aperture or DOF preview, the metering might be screwy especially if you try to link it to a focus point, no autofocus, you probably won't be able to use auto modes well (yeah i guess you could use aperture priority), and as far as i know I'm not sure if the diaphragm sizes on the FD system and the EOS system are the same if not similar. And then the factor that you have a 1.6x crop comes in too.
there's some canon primes you might want to look at. move around on that site and you'll get to some reliable third party glass.
motherdrizzlinSL: no, it was disturbing. I thought my fecal matter was going to retreat back into my bowels.