Just take like 3 levels of Ruby Knight Vindicator. You get no ACP to sneaking with that, and then you can add silent moves, greater for an additional +15.
Anyway, if you take mithral breastplate, it's only a +5 max dex and +5 AC. In my opinion, if you want armor and you HAVE to have it be light, maybe 'cuz you're a swordsage, I like giving myself a set of +1 Halfweight Mechanus Gear. +11 AC bonus, no ACP on sneakin' if you're a RKV. And then you have, like, a whole other stat to put things into, say, wisdom.