The simplest is to drop a tooth off your front sprocket.
Everything else is either too exspensive or of little value, although if you can find a cheap Staintune it has made a noticable difference to my F650 GS , in throttle response if not in peak power.
The F650 has plenty of power for its intended use anyway , so if you feel the need for a lot more you have probably bought the wrong bike.
Just dont get blinded with one figure , peak HP - in everything other than a traffic light drag race there is not much gain between the 50 hp of the Beemer and the claimed 83 of the Suzuki Wee Strom , and the GS feels more responsive up until 6500 revs .