Currently going back and forth whether or not it's "worth" replacing the TSX's 6th gear (0.659) with a 2012 Civic Si 6th (0.647). The difference is a miniscule 1.85%, but the difference in RPM looks a bit less trivial. I already have the TSX gear.
Honda Insight stock transmission: 2217rpm @ 65mph
TSX 6th with 3.4 FD: 2181rpm @ 65mph
Civic Si 6th with 3.4 FD: 2141rpm @ 65mph
Total cost adds around $200, maybe less if I plan to replace the synchros for 6th anyway, for a 40rpm reduction:
https://www.kseriesparts.com/HON-9THCIVIC-6TH.html
For comparison, moving to 185/65 tires (which I'm very likely to do eventually) would net a decrease of approximately 100rpm / 4.9%.
Tell me I'm crazy. This adds about 2.5% cost to my budget for a <2% RPM reduction on the highway. For comparison, my final drive affects all gears by roughly -30% and is around 15% of my budget.