In Jason's review of Rock Band 2's peripherals, he called the updated instruments not a necessity but "a bonus for people who were patient enough to wait for Rock Band 2." In other words, the new instruments were all around nicer to play than the old instruments. But what about the pesky sound output? My wife drummed so loudly in Rock Band 1, I'd have to put on headphones to play (she just rocks too hard for my old lady ears). Joystiq has done a quasi-scientific decibel analysis of the new drums and guitar, and they've found a moderate improvement there, too.
Striking the guitar frets was quieter across the board (from light to hard button presses) on the new guitar with an average drop from 12 to 6 decibels. As for the drums, you can see from the graph that most of the strikes were quieter, with a strange anomaly occurring in the hardest strikes that actually made those hits louder.
You can hit the link for the full analysis (with graphs!!), but it looks like everyone but the most aggressive drummers will find the new Rock Band instruments quieter. Sorry honey. [Joystiq]