Having just gone down the road of removing the Y-piece and replacing with a full Remus system, perhaps I can add a bit here..
first off the front (Y-piece) is fairly easy to fit, anyone who has a little mechanical ability should be OK..be carefull unplugging the Lambda sensors though, they come apart quite easily!
drop off the 'can' and then disconnect the front section, watch out with the strap that goes around the Cat, I undid mine completely and the Y-piece came away without to much hassle.
the Remus Y-piece bolted on very easily, the quality was pretty good, although having had them before I can say that they discolour pretty quickly! need to keep on top of them with some Autosol.
Fitting the can...Hmmmm, well I got the can bolted on and did that standing back and adimiring my handy work...great I thought...must go a ride it to see what it sounds like...sounded bloody loud in the garage, and not wanting to gas myself...I thought it best to do a road test...
after about 40 miles or so of rolling off the throttle and then gassing back up i decided that I liked the noise, reminded me of the 1200 racing at Le Mans this year...(a bike that could be heard above all the screaming fours) cue the best comedy moment for ages...I had not noticed that you should also fit the metal insert from the original can...this spaces the can out by 10mm or, if you dont fit it, you do what I did...you melt your number plate and blow hot molten plastic all over the back of your shinny bike


spacer in all good...new number plate as well.
as for power increase etc...well its like this..it sounds faster, seems to have a bit more drive after 6000rpm, and is a lot lighter than the stock system, I got my brother to ride it up and down the road, did a couple of slow passes and a proper fly by, just to see if it was properly loud...it was a bit on the fruity side when pulling hard, but if you potter along its not offensive at all.
my bike has only got 2000 miles on it, so its not anywhere near loose yet (my old GS didnt loosen up until it had 15000 on the clock) so performance wise I realy dont think its much different to stock, as I said above just feels like it tugs a bit harder at 6000rpm..
hope that all helps...FW