There is no .flags = PLATFORM_FLAGS_DECOMPRESSORS for *-efi targets. Hence,
"grub-mkimage --compression" flag is ignored for *-efi targets. Is this
> 2) Not compress decompressors and their dependencies.
Did you miss the "most (gzip)" and "adding gzio or xzio to modules list in
grub-mkimage" remarks? It means that gzio module is not compressed, and that
some modules decompress fine when loaded. Others result in "error: invalid
arch independent ELF magic".
> 3) Use only supported compressing options. Defaults should be ok.
Did you actually test any compression option with *-efi targets?