Sure, it's just a name (and GC NKit-ISO is functionally equivalent to GC ISO = GCM)
Define "performance" and "compressed" - NKit-ISO isn't compressed, "just" scrubbed and decrypted (that's why it can offer the lowest sizes after compression with a general purpose compression program) though of course they can't then directly be used;
as said above there's no performance difference, for GC titles, between NKit-ISO and ISO since nothing is actually moved around the disc (unlike less accurate GC "trimmers");
NKit-GCZ requires of course more processing power to play directly since it has to be decompressed on demand, but that's not significantly different from a regular GCZ
That answered everything I needed to know, thanks. I wasn't entirely sure if the shifting around of the data into a smaller image would introduce any reading differences, particularly with Wii games.