tamron mc7 or other teleconverter?

Your images are surprisingly good for a TC, a bit of heavy stop down though. In real life to use smaller aperture and faster shutter speed (due to extended focal range) your camera ISO will be pushed quite high. I have Kenko Pro 300 1.4xTC (one of their best before they stopped making Sony version) I can get it AF with my 70-300 and 70-400 lens in good light. The result? It offer no advantage over bare lens image upscaled. TCs are designed to work with fast prime lenses. My Kenko works wonderfully with 135/1.8, at 189mm f2.8 image is tack sharp, you will never get that kind of performance from a zoom lens.