EFI variables are not supported on this system.
I'm thinking your statement of:
"...possibility the Mytek kernel will not boot properly on your setup if you installed Ubuntu in EFI mode (i.e. with CSM off)" means my system actually should boot properly with the Mytek kernel. Now whether installing it in that mode is different than running it in that mode...I dunno, but I'll bet they are the same.
Ok. Onward!