I'm running into the same problems, both in RB3 and The Beatles:RB as well.
I've tried using cIOS rev's 20b and 21 (bases 56 and 57) and even on a whim I tried Hermes 223 v4 base 38+60 (saw it on another forum). If I use Hermes cIOS 5.1 base 57, the DLC will load properly with no lag whatsoever, but my RB2 drums won't sync (I use a GH:WT guitar so it's fine). Using a base 56 cIOS will let me have drums, but again the DLC lags. I wish there was a way to have Hermes 5.1 with a base 56, but I suppose I'm just wishing. If anyone has another suggestion, I'm open to it. If need be, I'll buy the game and run from disc, but I like using the usb loader better anyway.
Another option I'm considering is perhaps purchasing a WODE. If anyone has one and has used it for any of the RB/GH games, I'm open to your opinions of it.