[Excaliber]

This REALLY sounds like the main jets are just a little bit to small. When you "step on it" what is happening is the power valve opens. Thus enriching the system. When your main jets are "dialed in" you can STILL feel the power vavle respond but the hesitation and slight sputtering of which you speak goes away. I move up by two steps on the main jets. Sounds like the power valve is just about right. It SHOULD kick in with a slight increase in throttle application.

Bear in mind when the main jets are just about perfect for cruising, no sputter, smooth, they will be on the lean side when you remove the air cleaners and/or sidepipes. Normally you would change jet size by 4 or more points, but it sounds like your pretty close to correct size right now.