Without changing the programming of the PCM to lower the temperature that the cooling fans come on, I can't see it having much affect in stop and go. There would have to be enough wind coming through the radiator to keep the water circulating at a lower temp, and this would seem only available at highway speeds.
I believe the programing of the computer varies greatly below the operating temp that is maintained by both the thermostat and radiator cooling fans. I don't know how this affects the a/f ratio or mileage. There is an active thread on the SRT-4 forum, and it makes some good points for and against the lower temp tstat.
If you are doing a lot of freeway driving, it should be effective. In everyday driving, I don't think it would give much help unless the PCM was programmed for the lower cut on temp of the cooling fans.
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