These ones: http://support.amd.com/en-us/download/desktop?os=Windows 8.1 - 64 But you already seem to have the latest stable AMD drivers. There are newer beta drivers if you feel adventurous. Are you having trouble with PA with your current drivers?
When does your performance drop? During multiplayer or single player? What do the stats say when you cycle through them using <Ctrl>+P? (Specifically, the FPS.) Does is start immediately in the beginning of the game, or only much later? Does it happen only on large systems, or also the really small ones?
Immediately, single and multiplayer, all system sizes. Frame rate 2.5 Crashes in lobby (not completely, just not responding,) lags entire computer
Wow. What changed since Tuesday to get PA in such an unplayable state? It wasn't like that before, was it? In any case, you might want to check the list of bad software that can cause stuff like this: http://steamcommunity.com/app/233250/discussions/2/558751813508748383/
So nothing from that list, eh? Well, you can try some of the tips that are outlined here (if you haven't already): https://forums.uberent.com/threads/how-to-improve-performance-all-video-cards-driver-links.53165/