You arent getting it.

Non return fuel systems DONT FLOW enough for high performance applications.

Yes, you can make as much pressure as you want but it doesnt flow enough fuel. When you're out of fuel flow, you cant up with pressure to close the gap.

Higher pressures means lesser flow. Thats the problem with nonreturn systems, you have to run such high pressures that you lose flow. With a return style system fuel is constantly running through the rails, with a returnless, it does not.

Return systems flow more, which means you can run a bigger injector @ lower pressures, which is more reliable. Not to mention, the less you compress the fuel, the cooler the fuel is.

I'll type more on this once I sleep