Yes, WoW now takes a lot of ram on initial load, and then settles down after a few minutes. We don't know why, but it was something new introduced in a recent-ish patch. Could it have been after April. No idea. Not tracked.

However, different results on different PC's is not completely unknown. Different hardware may invoke a different response from the game if there are expectations that certain settings may be used based on the hardware detection (conjecture). There may be differences in the configs for the games. There may also be different background programs, driver addons and other things loading that are all contributing.

Can you 10 box on 16GB. Probably not. The budget is generally 2GB per instance now.

Moving WoW to your HDD will not help.
Be aware that some WoW graphics settings will change due to a patch (which you no doubt received), and sometimes the Anti Aliasing settings will turn themselves on when you change other settings, so those are worth checking and double checking.

For more ram, it is better to stick with similar to what you have. They wont be a "matched" set, but if the same brand, series and MHz, the chance potential for mismatch issues are very low.