I remember back when people mined scrypt some dude made a great deal of a modded BIOS. One of the things I remember is that yes, he said you can shut down whole GPU blocks. The power consumption was greatly reduced - oddly, as the whole point of using compute is to bypass ROP units.
But I don't know if you disable them 'forever' to next reboot or what. I certainly wouldn't do that for cards which are not dedicated to compute only as shutting down the ROPs will make the hardware unable to run any graphics API in existence.