I think it's fine at about 6k right now. At 7k last night there were a few small spikes (1-2 over about 4 hours of playing) which is perfectly acceptable at peak.
Any cap increases should be done at max level though. There's already enough questing competition, especially in the more populated area like STV/Tanaris.
There should probably be no more than about 8k players on a single realm anyways but hey, all I care about is PVP so if there's 10k with no lag, then overpopulation doesn't affect me.
Go for it haha :D

