Leaving any AV on right now shouldn't impact gaming. Quad core CPUs with 4-8GBs of ram is the norm now. I could see issues way back when with single core CPUs and 256-512MBs but not now.
Running an active scan might cause issues. We have enough CPU and ram power to not have a scan impact performance, but hdds are a different matter. I can easily see a single 5400-7200RPM drive have issues loading a game with a full scan in progress. But just leaving even a bloated AV running like Norton or McAfee shouldn't be an issue.
I'm not sure how exclusions work but I'm guessing it won't matter much. Spinning hdds just can't transfer data or move the heads fast enough. So if you are running a scan on the drive while trying to load a game or level it will be a lot slower then it should be. (read heads needing to be in two places at once.) Having the scan run while you are sleeping would eliminate this problem.
used it for years and i dont exclude anything. i get the fps i should in bf3 and every other game i play. it has 0 impact as its so small it can scan every file i load without it affecting system performance.