As far as top speed goes...and with your test that didn't quite get to top speed...what happens? Does it "spit" or backfire, or any sharp noises (lean) or does it make a duller stumbling, blubbering noise (rich)?
I was hoping for a more radical change than what you experienced.
There is a science to reading the color of spark plugs, but just for the heck of it, pull it out and see if it's VERY white or dark brown.
If the bike starts well, and idles well when warm...probably your valves aren't too bad. Compression though is another thing.