I think the easiest fix for cross server sage would be to base the brackets on power instead of level. There is a major difference between players of the same levels. For example I am a F2P lvl 90 with 61k power, and on my server that is low for a lvl 90. There are players with 15k - 60k more power than me. So basing it solely on level does not make a balanced sage war.
Basing it on BP instead will match players together that have similar BP. Yes there will still be players that have an advantage over others, but the advantage wouldn't be as massive.