Just to make things a bit easier for you, I have tried the FA 50 with the Tamron 1.4x TC (one of the better TCs out there), and found that it's not really usable wide open. In fact, it doesn't become usable until you stop it down one full stop, so you effectively end up with a 70mm f/2.8...which is what your Sigma is
It might focus quicker than your Sigma, but probably isn't ideal.
If you didn't need autofocus, I'd tell you to get a Jupiter-9 85mm f/2. It's very usable at f/2, and plenty sharp by f/2.8. But if you're going to focus manuall, you can do that with your Sigma.
Do you need something faster than f/2.8? If not, then let me throw you a curveball: Have you considered getting a Tamron 28-75mm f/2.8? I've heard a number of people say they ditched their 70/77/85mm primes since getting this lens. For a fast zoom, it's not even that big or heavy. And at around $375 new, it's a steal too.