My understanding is the R# is simply a casting/forging number and has nothing to do with the grind on the cam. My first hand experience indicates the SC's were definitely faster than subsequent bikes for most of the reasons previously mentioned. The intake ports in the heads were hand finished (major difference). No seals on the exhaust valve guides. Full floating rocker arm shafts. The cam was hotter. The spark advancer had a different curve. The exhaust had very little restriction, especially with the baffles out. The carbs behaved differently even versus K0's. The smaller front sprocket made a big difference. These early bikes probably made more than the claimed 67bhp and like already mentioned, would exceed redline in 5th gear.
I think I have already told this story but here goes again. Eight of us rode to Daytona in February 1970 for the BIG race. I was allowed to ride the 1st SC we ever received (don't know the VIN) as it was in the shop having been traded in on a K0. There were 4 other CB750's, all K0's on this ride. The SC I was riding was clearly faster than the other bikes, from a standing start, to midrange roll on, to top speed. We were riding across the South on I-10 which was pretty wide open for long distances back in the day and we could run WOT for miles at a time and did. The K0's all topped out right about 118-120MPH. I could run right along side them rolling the throttle on and off. You know the wah ba, wah ba, wah ba thing. Screw the throttle open and leave them in my dust, running well over 125MPH on the speedo.
Having said all that, I did own an early K0 that was HOT. Maybe close to the SC. Also owned a K1 that was HOT. Maybe close to the SC. Like any bike or car for that matter, some are just faster than their twins. Break your new bike in like Honda says and you will have a slower bike, typically with glazed cylinder walls. Run it hard from day one to produce a good piston ring seal. The precision of the tune can make a huge difference - valves, ignition, and especially carb sync. I always use a mercury manometer to sync the carbs precisely. Let 5psi out of your tires and see what happens to your top speed. It ALL adds up.
Keith