I have recently fitted a pair of GPR cans to my 660 Tenere.
The cans came with 2 sets of baffles one set is street legal and the other race. The blurb says the street legal setup needs no adjustments to the fueling etc.
Obviously the first thing I noticed was the bark not bad a little loud but not bad and yes the engine was a little more flexible around 2500/3000rpm but at the bottom end there was a hint of the engine pinking but above 4000rpm there was huge flat spot and top end was way down on power so I fitted Kevs fuel mod and this improved things but you could still feel that the engine was not at its best. I then altered the baffle on the nearside can to created a bit of back pressure with in the exhaust and this made a big difference preformance now the fuel mod is set to 4pm and she is running much better, it is still work in progress. Once I am happy that with cans I will be refitting the standard ones to compare the two setups
So beware if you are making changes to the system you may end up with a lose of preformance