I had a race MC28 at 75hp ten years ago. This was on 100 octane gas with a fair few mods. It also had ram air, but that was never tested on the dyno because we couldn't make 100mph wind. My MC21 street bike made 74hp on unleaded a couple of years later. This was thanks to a couple of extra porting tricks I'd learned.

A couple of years ago I fitted RS cylinders and heads to an engine, running the NSR pistons and a set of special pipes. It made 76hp.

The RS (NF5) score well in the internals department as Andy says. everything is lighter and more precise, with lower friction.

The porting of the RS can be closely matched on an NSR, but you'll never get it quite the same because of the NSR's ropey RC valve design. The RS is much better and fits nice and flush with the port when fully open. I spent hours welding up and dressing the NSR RC valve for the 'perfect' fit, but it wasn't good enough. Did gain a pony at the top though.

Basically, The RS is a better design (for it's intended purpose) than the NSR, and as such makes more mojo.

The NSR's much better in traffic jams though.