Exhaust Leak?

I would check for any exhaust leaks maybe by making some very soapy water, putting it all over a cold exhaust, get the vrou to stand by to help look ,start and check for bubbles.

I also had a loss of power and top end. Took it in for a service during which they adjusted my valves and in fact reshimed my intake valves or something to that effect. Like a new bike now.

But I would check your air filter to start with, even a slightly clogged air filter will affect the performance. 

Hi Late (but valid)

A quick test for an exhaust leak is to close the exhaust outlet off with your hand.  You will hear the 'hiss' at the leak.

NOTE!!!. Most 650GS's leak at joint between the left and right muffler boxes (in middle of bike above rear wheel.) as ther is no clamp (on Mike's old and my facelift) not even a 'crimp slot' to indicate that a clamp should be there. There is also black soot marks to indicate the leak.  Clover that with a wet rag and do the 'pressure test'  Any other leak will hiss (only possible places is jount at left pillion footpeg and engine mount.)  There are no gasket where the exhaust is clamped to the engine, only a thin plate with lugs to keep it in place.  Check the tightness of the two nuts.

PS.

In the old days I blew a few silencers (that were on their life limit in any case) with this test.  The pressure should blow your hand away even at idle or enough pressure should build up to stall the engine at idle.

Adie