I leveled a BE mixed team to 70 and leveled a shaman team to 80 without RAF, all old school leveling, on Thaurissan. The population imbalance was on Thaurissan ever since I started on the server. Seeing Alliance while leveling is like a system shock. I feel so bad about the population imbalance that I don't even gank Alliance in fear of them getting frustrated and quitting / transferring.

So I doubt very much that the faction change option made any real impact on population imbalance.