I just bought two QSFPTEK - QT-SFP-10G-T 10Gbase-T SFP+ modules.
They are 2.5/5gb capable.

I have one installed in my CRS308 and another in my CRS305 and both are running 7.8. I bought these because i just purchased an eero 6E mesh set.
I have them plugged in and they work. However, both switches tell me that they are running at 10Gbs The app to manage the access points tells me i am running at 2.5Gbps.