i believe the fps drop is due to the shadows. log into a single wow... go to video settings and set the shadows to the lowest level. then exit to force the setting to be written to disk. then start your wows normally

and no, you cannot run in 'full screen' mode... but you can have your wow be the size of your full screen using keyclone's maximizer on one monitor.. and a 2x2 grid on the other monitor (this is how i normally run mine)