@Huckle MPS looks like PCIe max payload size. There are a few discussions about whether Linux should set it or the BIOS, it looked like they decided to let the BIOS set it. You might check if the BIOS is up to date on the motherboard and if there's any sort of upgradable firmware on the card.

It works with offloading disabled, I really don't think MPS is the issue. Have you tried disabling each offloading option individually? Maybe you can find a sweet spot where the bugged option can be disabled while keeping enough to get your throughput up.

I would be inclined to switch to 3 network cards if that's an option at all. Without the offloading you're probably pegging the CPU to even get the 270. That means if anything else is using CPU it will drop below 270.