It's all relative. You'll be able to handle more weight with a proper, arched strength and conditioning form. But it simply comes down to biomechanics. The flat back bench press is simply not a healthy movement (This is if you actually do full ROM and touch your chest)... What we see with people who bench press with a flat back is, they compensate by only going half way down.. To avoid the bottom ROM of the bench press which puts a lot of pressure on the pecs and shoulders... So essentially, most flat back bench pressers do exactly the same ROM as those with the proper S&C arch.
Also more benefits to the arch besides just injury prevention and health... Relating to the movement biomechanics... Better loading on the triceps and chest.. The arch forces the chest to stick out and the shoulders to retract back.
Honestly, I've never met a flat back bench pressers who could bench 300+, UNLESS they were clearly on heinous amounts of anabolics and half repping in a constant tension bodybuilder manner.
ALSO: Don't confuse close grip bench pressing with regular bench pressing. Due to the nature of close grip, elbowed tucked in, it's much easier to tolerate a lack of an arch. If you can bench heavy with a flat back fine, you are most likely benching with a relatively close grip, and or not touching your chest.
Also will say this in relation to why flat back benching sucks. Due to the shoulders not being pulled back, you won't be able to hit the chest/get tension on it, so you have to compensate by widening your grip and flaring out the elbows. This provides a whole new dilemma.. Essentially shoulder impingement occurs... Think of that side delt raise top posiion, now put heavy bench press loads on that... Not smart.
When you break it down, it's a no brainer why I recommend to always been on the safe side with an arch when benching heavy... And as I said... It's hands down better to have a more excessive arch, than to have a flat back/no arch.