There are 3 possibilities that I'm aware of.
1. the 3.0 ratio FD from a european R850 RT. Available from your dealer.
2. Switch to a GSA, or replace your trans with the Enduro trans from one.
3. Go to "the Dark Side". You can get a slightly smaller diameter car tire that will effectively lower your drive output. Considering tire wear on a rig, this option will save you money on tires in the long run, and don't cost any more than the 3.0 FD
None of the options will get you more than 3-4% lower. A lower profile car tire, with an Enduro transmission and a 3.0 FD will get you about 7-8% lower (which is a huige difference)
Jay, from DMC sidecars once told me he has only wished for a lower ratio when towing his tent trailer behind his rig, 2-up and with luggage.