You always find that smaller motorcycles are really crippled by EU restrictions at certain revs. It's only really at these revs that you can find extra power, usually by rejetting or using an FI mapping tool. Outside of these areas you will only find gains by significantly increasing the mixture flow through the motor, and manufacturers are very clever at finding those limits while meeting noise regs. Indeed in the last few years manufacturers have been creatively using the backpressure caused by a restrictive, road-legal exhaust systems to provide a torque boost across the midrange, but at the same time using the resonant frequency of the system to keep peak power with the competition. The net result is- you plonk a can on and lose all that careful tuning. As Desmo rightly says, the only way to really make big gains on the most modern 600s is a full system,, rejet/remap, and some quality time on the dyno to set it all up.
Remember the manufacturers are competing, and the 600 class is the most closely fought product area. They all spend millions developing the motor which makes the most power, but retains usable torque while still meeting emissions and noise regs. They really are getting very good at it, and the days of sticking a £150 can and rejet kit on your 600 and getting 10bhp right across the rev range are well and truly over, unfortunately.
I'd recommend the Quill T3 cans to those of you that want a decent soundtrack (but legal at the EU testing revs), but don't want to fiddle with jetting and FI, and risk cocking up the delicate power balance that the manufacturers have built into the bike.