I have never used it with a TC17e, only with a Tc14e on my D2x. Speed was OK as far as i remember, but I didn't use it on difficult subjects AF wise.
In order to get relevant answers, you'll probably have to tell us what camera you intend to use the combo with.
If I were you, I'd try to play with one in a shot before buying if you have doubt.
Regards,
Bernard