The biggest advice I could give you if you wanted to try to see how low you could tune your guitar without having to do much truss rod work... I suggest getting a set of Zakk Wyldes GHS Boomers... He has two different sets... One set is for half step down, drop d, and standard tuning.... The other set is a bit thicker and is for anything lower than that. So i suggest picking up a set of those. You'll have to put some more tension on the truss rod and possibly grind out your nut a little to accomodate the large gauge... But they sound awesome and it should give you enough tension to tune low without getting a full setup.

I don't recommend tuning your low strings down and leaving your high strings in standard... This causes a torquing action on the neck and can lead to twisting of the neck... and in extreme cases cracking or it can push frets out..... If you don't need them... I just suggest tuning them down a half step or so.... The torquing won't be as severe that way.