If you must have the gears, I'd see about having Bruno or one of the final drive shops do it for you. The gears are very particular with regard to shimming and bearings. Also, is yours a paralever or monolever? The R12C is a paralever type pinion, so you would need to upgrade the swingarm. Fairly easy to do though.
The K100 is a bit buzzy, unlike the k75 (which doesn't vibrate, for all intents and purposes). The K engines are built like aircraft engines (the blue and white roundel is a spinning propeller on a blue sky), meant for high RPM, high throttle all day long. Autobahn use after all. I rented a triumph thruxton when I was in Germany, it did not like the long autobahn runs (90 miles/45 minutes between fillups, 24mpg). Could smell the blowby, even at speed.
I wouldn't worry about the gearing. The extra gas you save likely won't make the swap worth it. I considered going with an RS FD on the K75, it probably isn't worth it.
The 32:10 gearing is exactly 80mph (calibrated, not stock speedometer reading) at 6000rpm.
You might find that you overgear the K100/K75 with the 2.54 gearing. There's a big frontal area, and 90mph at 5000 rpm might be too much. Would definitely need to downshift often.
I think the K75 would do fine with the RS gearing (2.91), but that's about it. If I replaced my FD for other reasons (not likely to happen anytime soon, it's a bruno's rebuilt unit), I would probably go for the 2.91. That would turn 5200 at 80mph. Probably bump my fuel mileage a little. But otherwise, I'll just run it fast, it doesn't care. German vehicles are built such that engine operation is permitted at any point up to the redline (or a soft redline, usually 500 under true redline), for sustained periods. Also keep in mind, peak torque on a K75 is at 6500 rpm, and for a k100, 6000. That is where it is designed to cruise, due to peak volumetric efficiency.
