RJ, if you havnt had your carb off and cleaned it yet you really should. A bitch the first couple of times but it becomes a lot quicker the mroe you do it, it could cure your top end problem a little and also make it easier to start.
RE: using the decomp to start it wont make any difference to whether it will actually start, it just makes it easier to start pedalling. The egine will still start if you dont use the decomp it will just be harder to pedal.
I dont know what the mileage is on your bike but when i got mine (had about 6000 on the clock) the exhaust port was absolutely clogged with carbon. So, its worth taking the head off and having a look down in there. That would definately hurt the top end.