Hmm... I'm doing up to 8 render to textures in my game. I just tried making the copyrect() part of the process only happen if you aren't pressing CTRL. It makes no noticable difference to the framerate either way. Perhaps the results will be different on other cards.
This system will still be useful if you want to render into a texture which is bigger than the screen.
This system will still be useful if you want to render into a texture which is bigger than the screen.