I build the way described in my wiki at least 5 times, and the make clean is not needed by default (unless you build it once before without the patch to libxine). In any case it can't hurt to add the make clean in the wiki.
When you say that h264 is playing much better, are you still playing them in xine without Coreavc, or is Coreavc now kicking in? You can check it by looking at top while playing a movie. If it's using coreavc, it will be the dshowserver process taking the most cpu, if it's not using coreavc, it will show xine_player using the most cpu.