As Andy says, it's all down to design.

The stock NSR heads are ok for basic shape, but they suffer in two areas. First being the squish clearance too big, and the second is the plug position.

The VHM NSR250 headsets reposition the plug to the centre of the head. This helps for complete combustion. However, in my opinion the squish is still a little loose at a shade over 1mm. They're 12.6cc volume which is good for pump fuel. If you reset the squish for 0.7mm (0.3mm thinner base gaskets) this would give you a volume of around 11.9cc, meaning that you'd have to run minimum 98 octane, or there abouts.

The VHM heads also offer better cooling (very slightly) and are more efficient, so I've found I could run leaner jetting. Be sure to check though before shuffling the brass.

Jeremy's got NSR300 heads, with inserts for both pump fuel and rocket fuel, so it'll be good to see how he gets on. Tread carefully brother Jeremy!