Hi OP..I myself have a cruiser 4stroke with the 4G timing belt drive. No jackshaft or fancy gears here though. I do have some thoughts: So far, I like the mellow range of rpm and speed that 2/3 throttle gets me, 22-25mph as above that the rpm shakes the whole bike. I don't know yet what exact rpm it is, but the HS motor definitely likes cruising below wide open for sure.
Thats fine with me, as I like the simplicity of one speed for reliability and less wrenching, but I need to make a big upgrade path choice concerning driveline soon. I've posted some pics that perhaps could be of interest; check em out!