You will loose some image quality using a tc. That's to be expected, but exactly how detrimental it is, is very dependant on the tc. For example, you would expect there to be less quality loss using a nikon / canon top end tc as opposed to a sub £100 Jessops special.
The other thing to bear in mind is that tc will halve the effective aperture as well as double the focal length. With the lens you mentioned it will become a f11 lens.
You will loose autofocus, most cameras are designed to handle f5.6 or so at the most with some newer af systems being rated to f8. There's manual focus of course, but best of luck to you if you want to try that, the viewfinder will be extremely dark. Couple that with very high shutter speeds (presuming you're using a crop body so the effective focal length will be around 900mm, I wouldn't want to drop much below 1/1250 sec or so) to eliminate camera shake and it all becomes very difficult trying to get anything other than a dark blurry blob.
My advice, don't be tight fisted when trying to get extra reach with your camera. It will only end in disappointment. Make do with what you have and save up for something more suitable, whether its one of the big sigmas or a f2.8 lens with a tc.
I used the Sigma 150-500 on a 350D for some time and it offers the best bang for buck. I'd see if you can pick one up secondhand.
