Yes, I am sure that you will get better performance when mining if you disable crossfire.

Crossfiring = 1 cable going from card to card and an on/off switch in the settings. Nothing to lose sleep over. Play with it when you get them and see what works best.

Search this forum for posts mentioning the same brand/model card as the ones you're looking at and see what they're doing.

What OS are you going to run?