I think you have to test for three things:
1. The glue from the sticker weakening the shell;
2. An increase in slip-resistance (the mount might increase the risk of the helmet rotating your head if it catches on something as you slide along); and
3. An increase in the risk of penetration by whatever is stuck to the helmet (go-pro mount) because the glue keeps the thing at 90 degrees to your helmet ready to be pushed-through on impact.....
3M did respond to requests for information from the ASTM (cycle helmets) in relation to their reflective tape. This obviously only related to the potential for adhesive to affect the shell strength. They said
"...many adhesives are available throughout the industry and it would be impossible to tell what the effect would be without testing the tape or sticker product on the respective helmet materials.... although we believe the 3M PSA retro reflective tape adhesives would not affect the helmet shells, because of the small amount of residual monomers in the adhesive, the use of all tapes/stickers should be tested to insure compatibility with helmet shells."
I understand that sticking mounts to your helmet is banned in (for example) Paris Dakar for safety reasons (item 2 and 3 above).