The biggest benefit of this ECU is that it doesn't check all the things that the newer PCMs check... like the presence of cats, evap canister, etc... so one is more likely to pass emissions with this ECU than they are with a stock ECU.
as for performance gains, i ran two back-to-back dyno pulls without changing ANYTHING on my 99 PCM vs. the mopar performance ECU... the difference in power was, literally, 1hp and less than 1 ftlb of tq.
The car is a little more responsive with the mopar ECU... but i can't explain why. power is the same across the band (with mine).
just my .02...
JD