The reason I ask is that on my motorcycles adding moly to the trans gear oil makes them shift noticeably smoother after 1,000 miles or so once the moly has "sunk in."

Is it worthwhile to do this on a S5?

 

Use a semi or full synthetic.
That gives it an "edge" over the regular Gear oil.
With the diff you can add an extra agent to the gear oil.

This question is almost like the "regular oil or synthetic" debate that seems to Never end.
Personally I leave it up to the owner and let them decide.
Myself I use a regular gear oil in the Trans on my N/A.
I use a Synthetic in my TII.

If you want synthetic..then go synthetic( cost being an option).You can always MIX the gear oil and make a Semi synthetic.,(2 1 ltr bottles of each).Basically that is what SemiSynthetic is.

 

I don't know about motorcycles...and don't really care. But it sounds like a bit of a halo effect to me. Properly chosen quality gear oil shouldn't need additives.
If you're objective, I doubt you'll notice anything dramatic switching to synthetic. Worthwhile...yeah, probably. Just changing is as important as anything IMO. But anything inside the transmission, synchros, forks, bearings, that's already worn won't be healed by it, synthetic or not.