Back in the day, a riding buddy went the whole Luftmeister route on his K100RS. Bike ran good until he spun a rod bearing at high rpm. Although these engines are over-built, I believe it is best to leave well enough alone with stock. If the bike isn't fast enough for you, maybe you need to go to a different bike.
The Luftmeister cams were stock pieces that were built up with weld and machined down. I didn't particularly like the quality of the finished parts.
Not possible with the K75, but I installed a used K1100 engine and Motronic into my '85 RS. Everything bolts up no problems. It is possible to install a K1200 brick into the K100 with a bellhousing change. The biggest problem to overcome is to adapt the K100 electrical system to run the Motronic system.
Frank