Two ECU's on the RST - Silver Ignition and boost control, Black fuel ecu.

There are 2 rev limiters - one in each ECU. When an ECU is chipped, it refers to silver ignition ECU, and most chips remove the rev limiter.

Its not as easy to remove the Fuel ECU limiter - it can be done by changing a capacitor inside. and this raises it to around 7200rpm. But, as it's an analogue circuit, and capacitors use a massive tolerance - you never know the true rpm limit if you do this.

The S1 has the ignition rev limiter, but doesnt have one in the fuel ECU.

The difference is in the wiring of the TPS - you need to bridge two of the S1 fuel ecu's terminals, as the the S1 ECU expects the TPS to earth through itself, where as on an S2 it is grounded to chassis earth...