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Usually smooths the performance, increaes midrange pull, some small BHP improvments - often if the boost is standard level the mapper will raise it to a safer max level ( under boost cut! ) obviously giving mpre torque & power.
Generally, a remap customises the ECU to the parts in the engine, which the masss produced ECU map doesn't.
I had an ecutek map on my old UK car when it was completely standard. It took me a few days to admit it was better but it was definitely an improvement.