Without a remap the exhaust mod would do jack ! I have rs6 exhaust on my allroad with free flowing back boxes, did it make my car faster ? No...remap, turbo and nozzles did !
Turbocharged engines have specifically tuned exhausts in order to retain certain back pressure and allow the turbo to have a transient response, by removing the "restrictions" you modify the exhaust path in a big way and the response changes accordingly ! Take all the results with a huge pinch of salt, a dyno is just a measuring tool, if you input wrong data it will output wrong results.