Doug,
You're thinking like a plonka .

KEEP your RS500 plenum, put four yellows in the usual place and four 1000cc ones in the back of the plenum. In normal running it has the fuel economy of a standard car, then at the point the yellows run out of injector time, phase in the 1000cc ones until you are only running these for hard use. You then have the four injector reliability (apart from the phase over point, but you would soon notice if an injector had failed as at full boost it would only run on three and at low speed it would only run on three. The SECS can run different types of injector AND the phase in can be done at any point and any split .