There are 3rd party TCs with AF. To use the AF feature, your true aperture after adding the TC must be bigger than f8, which is not a problem for prime at all.
I use a 7 element AF 2X TC with 50mm prime and velvia extensively, I couldn't see any obvious difference in sharpness under my 10X loupe. as compare with pictures taken by the same lens without TC.
However, TC magnifies the fault of the lens. For example, if your zoom suffers from pin cushion at the longer end, then TC will make it worse. Else if your lens sharpness is worse at wide open, then TC will often force you to use the max aperture on overcast days for handheld shots and when working with low speed film.
Sigh. There is a big difference in 2x converters. Sadly to say they do result in significant loss of critical sharpness and in particular result in contrast loss. There are unfortunately, no free rides.
Yes, a TC magnifies any faults in a lens. Chromatic abberations, coma, etc. But NOT distortion. With a TC you are effectively cropping out the middle bit of the frame, hence reducing the amount of visible distortion, which is always worse at the edges, which are cropped out after using a TC.