The upstream kernel has support for the PCIe version of the RTL8125B adapter, but I'm not sure about USB. If it's supported it will either just work out of the box we can add the kernel config needed for that. If it requires and out-of-tree driver, you are welcome to self-compile an LE image with it included. If that's not your thing, we're sorry, but we have been refusing to add more crappy Realtek drivers to our images for about 2.5 years now and have no plan to go back on the decision. Including more of their shitty drivers in a distro like CE which never bumps kernel version isn't much drama. In LE where the kenel bumps frequently and Realtek drivers break with every bump .. the novelty of hunting down patches wears off (some time ago).
NB: LE does not need "fastest" Ethernet speeds, although fast is never bad. We only need Ethenet to be "fast enough" and Gb is fast enough for 80GB disc rips, so it's not a high-demand topic. I've no idea about other 2.5Gb adapters, but we will have no issue enabling kernel config for any needed; under the same rules (in-kernel is good, out-of-tree is bad).