Correctness:
Dullishly test functionality of every line. Change `default.cfg` to the release lobby and bots, then everything gamelist, playerlist and chat related can be tested except joining and hosting games without OOS.
Completeness: One can look through any xmpp message handler easily before and after the patch and see that they do the same as before. I remember that I didn't delete any line but only moved them and added some iteratively. The Game class cache needs to be updated everytime one of its dependent values is updated, so one can go through all dependent variables and see if they are updated equally.
Going through this code several times makes one rediscover all the TODOs since introduction of the lobby, see the comments in the diff to see which things should be accounted here while rewriting and which ones have to be declared out of scope.
Performance:
Use Profiler2 and Engine.GetMicroseconds to measure performance and how often messages occur to estimate the performance impact and locate bottlenecks. See the analysis in the comments below and consequential optimizations.