We still have no idea what sort of 650 we're discussing here, but what's all this crap about straight line speed? Any numpty can twist a throttle.
Why not embrace the far more subtle and refined art of turning your bike into a handling demon, and yourself into a highly skilled rider?
First, install new fork springs, fine-tune the fork oil viscosity and set the right amount of sag using spacers (maybe $80 total and a couple of weekends of experimentation).
Next, bolt on some premium shocks ($200 and up for Progressive or Hagon, $350-$400 for Ikon, more if you want the full-zoot fully adjustable units.)
Somewhere in there, spoon on some premium modern sporty rubber -- Pirelli Sport Demons are the gold standard for GS handling. Maybe $220 a set, and you needed tires anyway, right? (The new Avon RoadRiders are also excellent, and a bit cheaper.)
You might also need to attend to the fork seals, swingarm bearings, and/or steering stem bearings along the way, none of which are very expensive.
Add in stainless steel brake lines and premium brake pads (anything but OEM).
Now here's the hard part: get the engine and carburetion back to bone stock (richen the idle mixture screws slightly), then LEAVE THE ENGINE ALONE. This means new o-rings for the carbs, seal the airbox, make sure the air filter is correct, and probably new engine/carb and carb/airbox intake boots. Set the float height very slightly rich, then fine-tune to achieve the smoothest possible transition from idle to 2,000 rpm.
You see, once the road begins to curve, Big Speed is not found at wide open throttle -- the secret is smoothness at low rpm, when you're just getting back into the throttle in a turn. The sooner and the smoother you can open the throttle, the faster you'll be. The amount of power matters only to goons who can only wobble through the turns then yank the wires open.
Of course, the final piece of the puzzle is you -- safely taking advantage of your bike's handling potential requires a high level of riding skill and committment to improving, maintaining and practicing that skill.
If you haven't already, take the MSF course, even if you've been riding for years. Listen, learn, and correct your bad habits to build the proper foundation.
Find some twisties and ride them as often as possible. Practice in parking lots. Program your brain with the angles when the peg drags on the right and the centerstand drags on the left. Ride with people who practice The Pace, not macho daredevils or pottering cruisers.
Article on riding The Pace:
http://www.ridehsta.com/html/safety.htm
Attend a few GS rallies and tag along with the group up front. Don't try to keep up and ride over your head, but pay attention and learn firsthand what these old bikes are capable of in the right hands. Try to attend a track day or three if you can find one nearby that allows vintage bikes.
On a motorcycle, 99% of speed is the rider, and maybe 1% is the bike. Very few riders are willing to actively improve their skills, believing that they were born amazing and immortal. Take care of some of the basics on your bike and spend a few years improving your skills. You'll gain far more speed that way than by trying to turn a GS650 into a drag racer.
Plus, it's a lot more fun to watch the goobers on CBRs, GSXRs and Ninjas get smaller and disappear into your rear view mirrors when you're on an old, heavy, and "slow" bike.