Sorry, but this has nothing to do with the problem now days. People were less knowledgeable during retail and characters came from progression (vanilla). I'm not even sure how the faction balance was during retail tbh (as in actual numbers), but again, it is a bad reference to compare to.
Every "decent populated" TBC private server have had faction imbalance with horde always as the higher populated faction, at least the ones I've played on (7 in total, 3 bigger ones). Even more so when it comes to smaller TBC PvP exclusive private servers. By bigger ones I mean around 2k population.
As someone already mentioned, faction balance has been achieved on a decent populated tbc server by giving alliance free lvl 40 mount. Not sure if I approve of that method but don't really have a great suggestion myself.
Maybe have a naive approach with faction queues 55/45 or even 60/40 and see how it plays out. The population for Warmane TBC might be huge and we don't really have references to compare to when it comes to 8k+ ppl etc.
Mabye it will be fine, probably not though.