What camera are you using?
I shoot 300mm with a 1.7 TC which gives me 510mm reach.
The main issue with this lens and the TC17 is the AF speed since the aperture drops to f/6.7. Contrast is lower, sharpness is down a bit. Of course, since my combination became really slow, I have to use ISO800 or 1600 all too often to keep decent shutter speeds, which also does not help with the quality.
Instead of spending 6K for a 200-400mm lens I've decided to go this route. No regrets. I just have to work a little harder on steadiness and a little more sharpening in PS.