I got a EVGA RTX 3070 XC3 Ultra in my M1, and there is a ~ 1cm gap or so between the GPU and Noctua NF-A12x15 fans below them, the 3080 XC3 share the same cooler, so it should be similar to mine in terms of space between GPU and fans below.
I have not been able to fit an standard 25mm thick...