Removing the bloat ware usually helps but one thing I noticed about the TF300 was that the ASUS updates are often responsible for some(a lot of some
) of the lag. My device was running very smooth until the latest update from Asus and then just like yourself it a bit laggy, GTA for me was really smooth (until the latest update). I've moved on now and took the risk of flashing a custom rom and I must say that the performance is much much better even more so that it was before, I've still a bit left on my warranty but I figured there was no point in being frustrated with the asus stock and it was taking away my enjoyment.
In terms of gaming, did you know you can attach a Bluetooth controller like ps3 one? For me this made gta much more enjoyable but in terms of the keyboard, I didn't really find that many games either, I put a number of emulators on it which most of the time support a keyboard an went back to the "good old days" of Atari ST and Amiga
If you like that kind of thing it will keep you amused for hours!