My thinking would be that a zoom TC will compromise IQ a bit at least. If you are using a prime then you are probably seeking high IQ so would be better sticking with a "prime" TC.
If you want a long zoom, can you put the TC on a zoom lens?
BTW I saw somewhere on here that you could stack TC's with an extension tube between them. I tried it the other day and the results were much better than I expected.