It has been proven to increase efficiency of the engine in its ability to get the exhaust gasses out. Not really any HP gains per say but, I do believe the bike would possibly pick up some HP. Just my thoughts.
Not seen bikes, but seen cars that've been dyno'd, before & after wrapped headers, it yeilded very minimal to no HP gain. Edit: Even most gains on race cars were attributed to reduced engine compartment temp., not helpful on a bike.
I wrapped my headers for looks with black tape, and after a few rides it just faded and looked like crap. No noticeable temp or power difference. Took it off and it was a waste of time and money. Live and Learn!!