Like Duckie said, unless the extra heat is causing you to back down your overclock or making your system unstable, who cares? You already have the two cards might as well use them
My roommate has the same system as me, except he has dual 7600's and I have a 7900. At the time we bought our systems, his dual 7600's were about 5% less total cost than my one 7900 after MIR, and he gets slightly better benchmarks than me. I think this is the exception to the rule. In my opinion, you'll almost always get more performance by buying a better card that costs 2x as much as whatever you're considering putting into SLI. There's exceptions, but IMO it's usually true.
But since you already have 2 cards, why not use them?
LOL I just remembered my roommate never got his MIR... They rejected his request and tried telling him he only bought one and made a copy of the IPC, even though he really bought two. Haha owned. (forget which company)