I hate giving out tips, but I just can't stand to read this thread.
Going to 400 expertise gets you a little under a 9% bonus, so you're losing a little more that 3.5% effectiveness. You can verify this in game yourself. All versions of battlemaster gear have them same stats without mods. Rakata gear has slightly different stats depending on what version you are getting.
To replace a battlemaster piece with a Rakata piece, you are either netting 28 mainstat and 13 endurance, or 18 mainstat and 22 endurance. Mods don't count, since you can switch them out. There is less of a difference for jewelry. so, switching out 6 battlemaster pieces can yield you 168 main stat and 78 endurance, 90 main stat and 110 endurance, or some combination in between. If you drop the jewelry pieces as you suggested, the gain is even less. So, to use your example, it would be:
BM Only: 12.45% expertise
or
BM + Rakata mix: 8.8 expertise + 110 end + 90 mainstat.