It's an ampacity chain : power supply, control station, contact rails, decoder
The power supply is up to you
Control station/decoder : SCX is limited to the stock motor (in Spain, they immediately fried a decoder with a Slot.it 25k motor) - I would guess the same applies to Scalextric and Carrera - logically, Ninco should provide something more for their NC5/NC6
Contact rails (continuity) : SCX seems OK with the redesigned connectors, Ninco have a good reputation in this respect, I read that Scalextric and Carrera have problems on longer tracks and require power taps/jumpers
Higher ampacity aftermarket electronic packages will no doubt become available in the future (I'm told hardware is not a problem) - I just do not see anything above 25k as being compatible with MCPL racing, the allowable number of cars is more of interest