So both output amplifier sections would still be driven, and therefore the result would be no reduction in output power capability, just a 6db reduction in gain as I and Atmasphere indicated.
This is correct. The problem of using RCA to XLR (apart from figuring out what is + and -) is that you are grounding one side of a balanced circuit. It will make your setup much more susceptible to ground loops and ruin its immunity to noise. Balanced means just what it says - everything is balanced with respect to ground and noise of ground loops (from power supply leakage, different ground points etc.) become irrelevant as they affect both line level signals equally.