It took me four years to stumble on your post, sorry I'm late!
8 track used 1/4 inch tape and split this into 8 tracks on the tape (4 programs).
In theory this means that the quality is probably on a par with cassette tape (1/8 inch tape split into 4 tracks).
Although eight track had a higher ips ratio, it had other issues, usually associated with head alignment.
Then then was quad8, a whole other story.
This guy explains things rather well: