I absolutely agree with jay, triceps are everything. I also think that for a lot of people, the stronger you are, the more you get out of a shirt, especially denim. My theory for why this is true is that everyone has weak links no matter how strong that person is, and if your weakest link raw will allow you to bench 300 and your strongest link will allow you to bench 325 then all you could possibly get from a shirt is 325.
But if you can lock around 500 out but a bum shoulder or something keeps you from being able to do that much at the bottom, then you may see a person who raw benches around 400 that is able to go 500 or so with a shirt, because for that particular person, the shirt takes out the weak link, and allows them to bench as much as their next weakest link will allow. Whereas if you have a person who is very strong raw and doesn't get much out of a shirt, then my guess would be that their triceps are not any stronger than their pecs, delts, etc.
Just my own personal theory.
B