I've noticed this too and I'm really dissapointed in AMD. I read other forums on sites like Tom's Hardware and they have had problems in the past. The only way this was resolved was through a driver update and update to minecraft.
I have Sapphire 7970 OC with boost, which is pretty much the same exact card and I've tried uninstalling minecraft and java and reinstalling it, setting specific profiles, using both the stable and beta drivers for my OS Windows 8.1, and nothing seems to work. AMD please fix the drivers for this game. You could run minecraft 20 times over on max settings with high quality texture packs. Please fix this. I'm only getting 20fps tops on the maximum settings. My intel intigrated 4600 graphics runs Minecraft better.