I have this case and regarding to the WiFi performance i am loosing 50mbit of what i had before i got the case.
First i bought the RPI5 and i had it without any case, getting a 250mbit connection speed during file transfers in my local network. After the installation of the RPI5 into the case i'm getting a 200mbit connection.
The RPI5 are at the same location as before, same with the router. No changes what so ever except the case.
Where you get your digits from a 74/75mbit WiFi speed is beyond me. It's way worse than it should be.
Now i have not tested my speeds with iperf, i have tested to transfer files (4gb+ size) over samba. I am USB booting my RPI5 from a ssd drive, and the end location of the file transfer is on a nvme disk (PC).