Why not run it lower if nothing else to reduce heat and simplify cooling.
BTW I got 24 5970s. 60W a piece = 1.4KW.

If my electricity was free I would still do it because that is 1.4KW less heat.
BTW phatk2 can run much lower than 300Mhz. I just noticed you said latest sdk. SDK 2.6 sucks for 5000 series cards and low memclock.