Before any remap, make sure your car & engine are up to date with regular maintenance.
IMO, it's better to get the DSG mapped at the same time as the ECU remap. It will cost more, but most tuners will offer a discount if you do both at the same time. The remap wakes up the whole car/engine, but to get the best out of it, DSG tune should match for the best launch strategy & shift points.
On the GTi, you might want to invest in an Anti-lift kit to get the power down better.
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