I have a FMF "Q" silencer on my 2001 RM 125. It is quiter, but not as much as I had hoped. I have not had an opertunity to DB test it with a meeter, so this is very subjective. When I purchased the bike used, it had a Bill's pipe and shorty silencer. I did not get the stock silencer with the bike, so I really can't answer your question (Q vrs stock). When I'm waiting to get on track, and comparing it other RMs next to me (with stock exhaust) it does seem quiter.
Compared to the shorty, the power difference "may" not be much (maybe a little loss in the low end). This is my first dirt bike so pre-Q, it had more power than I knew what to do with. Since installing the Q, I have sorted out the jetting (it was rich both before and after the installing the "Q"), so untill I switch back to the shorty for back to back comparison, I really can't say.
When I had the shorty on, my trail riding friends say they could clearly hear each shift I made from about 1/2 mile away. Now they can't.
On down side is to get the maximum silencing, they (FMF) say to get "Q" repacking from FMF. This is a "just for the Q" packing kit ("Q MUFFLER PILLOW PERFORMANCE PACKING 011394 $24.95" from FMF). It's a lot of packing pre-shaped that fits and works great, but is more expensive than using you standard packing. I'd like to get a db meeter so I can try it with different packings, and see whats best/dollar. I have not used std. packing in it yet.