Considering the result in the video below, it is difficult to argue with his decision. The performance certainly is there. His S52 turbo setup has shown 708 wheel horsepower but he states in the video he is running roughly 640 wheel horsepower during the race.
The E39 M5 does not have the fancy dual clutch transmission the M6 Gran Coupe does and the S52 six-cylinder has less displacement than the 4.4 liter S63TU V8 in the M6 Gran Coupe yet @Omni's E39 M5 makes easy work of the M6 once he hooks up.
Definitely impressive to pull on a stock M6 Gran Coupe in an E39 let alone one that is tuned with catless downpipes.