ECU tuning is common: owners report RENNtech Stage 1/2 and similar tunes pushing cars to roughly 600-650hp and making an S competitive with an R for modest money. But the community warns that aftermarket modifications crush resale on these cars, so tunes are valued for driving, not investment.
Tuning (RENNtech et al.) and its effect on power and resale
Mercedes-Benz AMG GT (C190) · ownership