Is it possible to use Ground Environment Pro textures for FSX? I guess those textures are at 5m res, it should speed up things a bit.

I heard yes. Somewhere, I don't remember. I haven got it pro, just regular, so I can't try, but like I said, I heard they were. Why not try backing you your texture folder, then copy the textures over from FS9 if you have them there. I tried it on day one with Activesky textures (which didn;t work), but just copied my original folder back, and all was well (if slow) again.