I know this topic might have been dicusssed but still if someone can have some real input in this topic, please do. Otherwise thanks for reading.

This is for a 958 non-turbo GTS

Car is stock and has the secondary cats bypass pipes installed.

I hear that the reduce in back pressure will lead to a slight reduction on low end torque. Although high end will benefit slightly.

I have called a local ECU tuning shop and they have promised 15% gains all round.

Not many GTS in my country so no real data here to look at

Can anyone here back this claim for the GTS?

Is it possible to improve performance through ECU tuning?