I think others on this thread have commented adequately on the possible power limits of flying over gross. The problems develop more insidiously if the power is adequate for near normal operations over maximum gross weight.
The USAF flew it's H-53 fleet over gross in the late 80's and 90's for a variety of operational reasons after the fleet transitioned from the rescue to the special operations mission. Unfortunately for us we did it backwards from the S-76 fleet. We did it, and then told the certification authorities what we had done. They came up with an inspection regime and then we'd go off and do it again. Not real smart on our part, I'll admit, but we never lost an aircraft to over gross operations.
Real problems happen when you operate off the charts. Working with extropolated charts worked fairly well for us for engine power and hover performance, but we were somewhat surprised after the engineering got done that we had also been operating out of permissable cg limits. The aircraft did crack where the engineers thought it would. It also cracked bent and wore out in places they didn't expect.
If lives don't require you to fly over gross, I highly recommend you don't do it, because lives are at risk when you do.