Today I got my remap done at HRED Motorsport and I'm really satisfied with the outcome
I also discovered that one of the previous owners had the car remapped in a weird way.
Here are the charts representing the weird remap, the stock ECU and the HRED remap :
I found the engine fuller on the whole RPM range and the difference isn't crazy like adding a turbo : the GT86 should come out the factory like that.
For those who wanted to hear the exhaust, this is for you