Quick version: M1 iPad Pro runs USB-C 2.5Gbe (and I assume 5Gbe) adaptors out of the box at full speed. Tested on iPadOS 14.7.1, but might work on earlier versions.
Longer version: I upgraded to the Xfinity 1200 Gigabit+ plan and put in a new Motorola SB8611 DOCSIS 3.1 cable modem with a 2.5Gbe port. I don't have a 2.5Gbe router (yet) so the only way I could test to see if I'm getting the full speed was with my 2012 MacBook Pro using a Pluggable Brand 2.5Gbe USB3 adaptor (the one that has USB-C, and also a normal USB adaptor dongle). Sure enough I'm seeing 1.4Gbps direct to the modem.
I knew the M1 iPad can do 10Gbe with a Thunderbolt adaptor, so on a whim I plugged in the USBC 2.5Gbe adaptor. I expected it to run at 1Gbe since I needed to install network adaptor drivers from RealTek on my MacBook Pro to get it to work at 2.5Gbe speeds. (MBP is limited to Catalina due to age).
Much to my surprise the iPad worked fine at full 2.5Gbe speeds, and I got 1.4Gbe straight off the modem into the iPad. I guess the drivers are baked into iPadOS now. Color me impressed!
/edit. I can't think of a use for hardwired networking faster than 1 gigabit on iPad, but it's neat that it works anyway.
Longer version: I upgraded to the Xfinity 1200 Gigabit+ plan and put in a new Motorola SB8611 DOCSIS 3.1 cable modem with a 2.5Gbe port. I don't have a 2.5Gbe router (yet) so the only way I could test to see if I'm getting the full speed was with my 2012 MacBook Pro using a Pluggable Brand 2.5Gbe USB3 adaptor (the one that has USB-C, and also a normal USB adaptor dongle). Sure enough I'm seeing 1.4Gbps direct to the modem.
I knew the M1 iPad can do 10Gbe with a Thunderbolt adaptor, so on a whim I plugged in the USBC 2.5Gbe adaptor. I expected it to run at 1Gbe since I needed to install network adaptor drivers from RealTek on my MacBook Pro to get it to work at 2.5Gbe speeds. (MBP is limited to Catalina due to age).
Much to my surprise the iPad worked fine at full 2.5Gbe speeds, and I got 1.4Gbe straight off the modem into the iPad. I guess the drivers are baked into iPadOS now. Color me impressed!
/edit. I can't think of a use for hardwired networking faster than 1 gigabit on iPad, but it's neat that it works anyway.