5GbE Not Much Better than 2.5GbE

I have been testing various 5GbE and 2.5GbE network cards for use with a couple of computers that only have PCI Express 3 x1 slots available. I am not using a switch; I am directly connecting between the cards with a CAT 8 cable. The 5GbE cards use an Aquantia AQC111C chip and the 2.5 GbE cards use a Realtek RTL8125 chip.

Copying files that are shared via SAMBA yield an average read/write of about 305Mb/second using the 5GbE cards and about 275Mb/second using the 2.5GbE cards. The read/write activity is between a 2TB Samsung 860 Evo SSD on the server and a 500GB Samsung 860 Evo SSD on the client. I also ran iperf3 to eliminate the file sharing protocol and I am still not finding a substantive performance difference between the 5GbE cards and the 2.5GbE cards.

While I don't expect to see double the performance, I thought I would see read/write performance close to the native SSD speed with using the 5GbE solution. I have not done any performance tuning and the MTU on each system is 1500. Is there any reason to expect I can get much better 5GbE performance by changing default settings, or am I expecting too much from 5GbE? At this point, I'm inclined to stick with the 2.5GbE cards because they cost half the price of the 5GbE hardware.