2 3000 packs, in series or parrallel or one at a time? If one at a time or parrallel, no problem. In series it's the same as 12 cells. As far as I remember, it cant handle that much. Check the manual at LRP , goto "download", where you can find it.
If you want to void your warranty then go ahead it will handle 12 cells as long as your gearing ok, but I don't know how low on the motor front you can go before it does blow. As for the fastest motor well a 6 or 5 turn comes to mind if you don't mind doing maintance every run. I wouldn't suggest using 12 cells with a 6 or 5 turn motor, stick with high turns like 12, you'll still be fast as hell.
If you esc blows up don't blame me(I predict it will).