I use the Kenko Pro300 DG 1.4x and find it very good (as good as the Canon Mk II 1.4x which is twice the price so I sold it!) and it'll work (don't necessarily recommend from these guys!)
Kenko PRO 300 AF 1.4x Teleconverter for Minolta and Sony
Don't know the Sigma TC but it'll work
Sigma 1.4X Teleconverter EX APO DG
And that's the good news! No probs with light meter working to set the aperture/speed but AF probably won't work (TCs will take the max aperture over the 'magic' F/8 limit where AF is usually switched off as the camera manufacturer reckons you're unlikely to get sufficient contrast for AF to work well)
Having said that, I don't know if Sony sets this F/8 limit and the "tape trick" will probably work to fool the camera (put a piece of sticky tape over a couple of the TC pins so camera fails to recognise the TC attached and allows AF to work - I use this on my Canon/Kenko/Sigma combo)