this seems to be a common issue on all Harley's, at least with ones I've owned. Why is it that my throttle has such excessive travel before full throttle is reached (yes the throttle is correctly adjusted).

If I warp it flat out - without taking a second grab on the throttle, my knuckles hit my forearm  

Is there a fix.......

like an aftermarket throttle grip with a larger diameter groove for the cable, see item #2 in below pic.

 

Or better still, maybe a smaller wheel at the throttle body.. see item #3 in below pic

It does seem like there is a lot of travel however it is only 1/4 turn. I don't know of any aftermarket throttle for HD's that is less than 1/4 turn.
Most jappers are the same however the relative position of grips to seating position seems to work better

I have not had this issue....I guess over the years I have modified the way I hold the throttle so that its not a problem.

If I am taking things slowly my wrist will be flat relative to the ground; however if I am in race mode then I'll actually hold the throttle grip differently with my wrist at an angle. Have a look at some motorcycle drag racing starts and you will see what I mean. I must do unconsciously now as its not until your question that I actually took any notice of how I operated the throttle.