How much overclocking is "okay"?
Manufacturers specify a maximum clock speed for a reason, as it guarantees correct operation under all conditions specified in the data sheet. Operating a device outside its specification might be OK in a hobbyist application, where an occasional failure can be tolerated, but isn't a good idea if reliability is important. You will have to test the system and see if it is sufficiently reliable for your requirements, under all circumstances.

I can't see 81 MHz causing a problem, but it can't be guaranteed.