I have several Faber LP 120's that I regularly overfill, and have never had a problem, I have filled them to 3800 psi.

I have friends that have Fabers that get filled to 4200 psi, and never had a problem.

The tanks are tested to 10,000 cycles at 4,000 Psi before failure, I seriously doubt anyone is going to ever fill a tank anywhere near 10,000 times.

The tanks when they are cycled are done hydraulically, and very quickly, which I think would cause more stress in them.

The greatest chance of a tank failure is when it is being filled (that is why I have my wife fill my tanks ), so I can't see anything wrong with a dive shop not wanting to overfill a tank.

I would never, never, never overfill an aluminum tank.