Out of curiosity, what would a bench shirt add on, say for instance, a 300 pound max lift?
Depends.
If you're fat, like Gene Rychlak, or Mike Wolfe, it will add extreme weights.
Fat in itself will add lots to a raw lift too.
I'm talking about Rychlak getting maybe 400-500 lbs out of his shirt.
Eg, now with Wolfe getting down in weight, I am positive that he would stand no chance to get 700 lbs shirted if he got down to 10% BF.
But at the same time, it's not as simple as adding pounds to a raw lift either.
It takes lots of practice to become a good lifter with shirts. The fatter you are, the easier it is to learn it IMO.
There are some ripped lifters out there who are able to get lots of lbs out of the shirts.
And then there are different plys. Single, double, triple plys. And canvas.
You could probably get 40-50 lbs out of a single ply shirt without too much training though.
In powerlifting, fat = rules.
YIP
Zack