afaik its was only the RS500 that had the duel injector rail, although the castings for the intake manifold still have the bits there (just havent been machined out).
Thats the bit i think i know - the bit I do know is this:
the reason for 8 injectors is actually for small power. back in the dark ages we never had super accurate/non leaking/fast acting injectors like we do now. If you had 4 really big injectors in the cossie it would run good on full chat, but would idle really badly as the injectors couldnt be controlled well enough to supply the small amounts needed at idle, whilst still dumping gallons in when needed.
So when its idling, or running low boost it just runs on 4 smaller injectors, and when the requirement gets bigger it kicks in the other 4, its all about controlability.
back to high revving:
having double valve springs is a strange one, its helps produce better power by making sure the valves shut properly and quickly but also increases friction.roller rockers can help a bit too by reducing friction.
even small things can make a big difference - such as titanium (or other lightweight and strong) spring retainers (the little collers that go over the end of the valve to hold the springs in)
anything lightweight and low friction will help, 6cyl is good, very stable and doesnt need a balance shaft for high speed and low vibration.
carefully specifying things such as oil pump and sump - so that there is just enough oil to let the engine run and with just high enough pressure to prevent losses circulating oil and prevnting splash in the crankcase. well shaped crank, alowing it to 'cut' through the oil in the sump without splashing and causing drag.
thats all i can think of just now