If you haven't adjusted your stance from longboard (F2 Lightning) to super short slalom board (Hypersonic 111) then there could be some real differences in harness line length, however I would think that you would want them shorter, not longer.
Also, "forearm burn" (your term) is more often associated with hanging onto the boom with a "death grip". Next time you sail, see if you can "play the piano" with your fingers while sailing at near top speed. (I.E. flex your fingers and wiggle them on top of the boom to prove to yourself that you aren't in the "death grip" mode.).
What size rig are you using (brand, sail type, size) on the Hypersonic?
How about a set of adjustable harness lines......?
Then you can very easily change the length and see if you need longer or shorter
lines (you can even change them between sailing upwind (shorter) to sailing off the wind (longer).
The Sailworks "Quiktune" Adjustable Lines are some of the best lines (easy to adjust, last along time if you use a Reactor Roller harness bar) on the market and only cost $48.00 if you buy direct from Sailworks.
Here's the link:
Hope this helps,