You are 100% correct, if your current brakes can make the ABS kick in, then its simply not possible to brake any harder.
Most fit it for styling (even if they think its for performance)
Only benefit bigger brakes have, is the ability to dissipate heat, so you can hit them harder more frequently - Which entirely depends on what you're doing with your van.
I have a set of Audi/Aston 4 pots (as used in the VAGbremtechnic kits the various dealers fit) but no point fitting them, as the OEM 340's are more than good enough for my use.