To address the prior DOT/OSHA discussion, there are industrial gas transfer safety regulations by OSHA that also apply to SCUBA and every other high pressure gas transfer application. They are written for manufacture, transport, and transfilling of all pressurized gasses. OSHA may fine a business for violations of their safety standards in these areas. Just because they typically do not enforce these rules in non industrial settings does not mean that they do not exist. Also violation of a known standard is a liability. I doubt that your business/personal liability insurance would cover a loss if something were to go wrong and it was proven that you ignored regulations and practices standards. I would like to point out that if there is a rare "overfill" failure that causes injury - The person overfilling a specifically rated tank will carry enormous liability in the civil court system. Especially if you have disabled the burst disks, or intensionally ignored proper process/procedure.
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I do have a problem with the mentality of "we always overfill and have not had a problem yet" attitude.
This is exactly like "I always drive 15 over the speed limit, and have not had a ticket yet".
Statistics state that given enough opportunities, the most unlikely is inevitable. Overfilling is a common practice, and modifying a known system outside its rated safety specs is a calculated risk, not something to be taken for granted.
I personally think that you should get a setup that is best for the application of use, your desired trim characteristics, weight capacity, and expected bottom time. If 120s work for you then keep using them.
I have both Faber LP95 with a "+" rating and x7 HP100 - the fabers are really heavy. I get about the same bottom time. And yes, I too get overfills - but am not cavalier about the risk.
