Its strange that some of you got problems with client checking md5 of files, for me all model edits, race swaps etc.. work fine without any md5 changes in wow-update-xxxxx.MPQ files.
Im running human male to goblin male and blood elf priest to nelf priest with better looking body edit, also nelf's body size has been made litlebit smaller, no problems so far.
All of those files are combined to 1 file, patch-1.MPQ that is residing in Data folder.
Yes, they are more specific than in wotlk, and all those teethpath's and files are case sensitive even .m2 file endings are case sensitive.
There is 2 teethpaths to files, one is "virtual" and one is by folder structure and names.