APR Stage 1 ECU will more than bridge the gap to your GTI
APR - High Performance Development for Audi, VW and Porsche Vehicles.
However you sound like you're wanting to acquire the files yourself (for free?) and DIY??? You'll find most stuff is encrypted and locked to a single VIN or ODB device.
Intercooler (from Audi S3 or APR) and carbonio intakes are also popular mods but make a much smaller power improvement compared to ECU remapping).
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