I'm gaming with avast! enabled for years and i haven't seen any drawbacks so i can't really see any reason to disable it. Most of the data is just passed through the scanner, that's why you won't even notice it and that's why it's simply pointless to even disable it in the first place.
You might see a difference when launching Adobe Photoshop for the first time because it will load bunch of DLL plugins. Games usually contain just few DLL files, main EXE file and loads of textures which aren't even scanned because they cannot contain malware. Seriously, like 3/4 of the game content in size are textures. The rest are misc files that are scanned quickly and because of the advanced caching, they hardly ever need rescanning.
My recommendation, just leave it enabled.