Playing with spec in sig @ 1920 x 1200 everything maxxed AA on i normally get 50+ fps. Some levels rape the fps. Specifically the one with the valley one side, town on the other with a lake in the middle with a path next to it. When in the valley at the top, when in sniper mode my fps drops to 10 - 15 but otherwise its runs pretty smooth whole time. I dont think the game is optimised for more than one core yet either.
Regarding the Maus, it does have some weakpoints (gun mantle, taking damage from track shots) but i think they are getting fixed in a future patch. Part of the problem with the maus, is it cant be played the same way an IS7 can. AN IS-7 is a very good solo tank. IS-7 VS Maus, solo battle IS-7 will win. If you put a maus with support + IS-7 with support, the MAus should win. For some reason everyone starts shooting a maus when it appears, probably due to its size. It can take a serious beating while your other tanks are fee to fire. Not to mention in city maps, when the maus cant be flanked its very strong.
Also bear in mind, the IS-7 has something like 60mm rear hull armor, where as a maus has a lot more as well as a lot of hp.