But PCIe is full duplex! With PCIe 2.0 x4 there's 4 lanes in each direction [1], so when 'forwarding' over a single 10G link you can expect to send and receive simultaneously at the speed I mentioned earlier.
Yah, I guess dividing by 2 isn't fair. But transmitting does impact receiving and vice-versa: when you're reading DMA descriptors, you need to wait/hold for posted completions, etc. It's not fully uncontended between send and receive, but more uncontended than a naive division by 2 would imply.
Yah, I said forwarding-- so each packet goes over the PCIe link twice.
So, to use your test-- 3.7 x 4 / 2 = 7.4gbps. I said around 7. 7.4gbps would be a bit more than I'd expect, but I'd not be shocked.