NK,
Welcome aboard!
What you are experienceing is the speed-limiter programming in the ECM PROM. The ECM will trim fuel injector pulses back to prevent the car from exceeding the speed rating of the tires. Usually, this is 118MPH, not 111MPH, but yout speedo head or VSS may be off a little bit.
The only ways to circumvent this are to disconnect the VSS so that the ECM has no speed reference for the vehicle (which will also set a DTC 24) or to install a different PROM without the speed limiter programming enabled at 118MPH.
I'm guessing that the RS has a TBI, and no TBI cars were shipped with 16" "Z" speed rated tires and the PROM with the speed limiter effectively disabled (actually, it's set at 254MPH - the limit of the programming parameters). If you have a TPI engine, you could install a factory PROM from a WS6/TPI car and not have the speed limiter at 118MPH.
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Later,
Vader
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"
I'm'a do Things My Way - It's My way or the Highway."
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