I big part of it also depends on if you have trained for the shirt or not
if your top end is not any stronger than your bottom end then you are going to get nothing out of the shirt
the shirt is going to assist you with the bottom portion of the lift. primarily the 2 board mark. there is definitely assistance in the 3 board mark and dpending on how broken in the shirt is, there is some as far as the 4 board mark but very little.
A lof ot the guys that are getting a ton out of the shirt just have top ends thatare developed FAR beyond that of their bottom end.
Myself, I train a lot of raw work right now, so I am only getting about 50 lbs out of my shirt. When I get closer to meet time (maybe 10-12 weeks out) my training will be more based around equipped lifting and I? will probably get about 100lbs out of my shirt.
It's true, some guys are gett9ing 200-300 lbs out of their shirt, but also what they are lifting is closer to 850+
So, if you're raw benching in the 3's to low 4's obviously you will not be getting enormous gains but you will more than likely see 50-75lbs out of the shirt early on and 75-100 as you learn the shirt and adapt your training to it.