Didn't KOY eventually go to the Motorola system before pulling out of AM stereo altogether?
As to my experience with it, the only AM stereo receiver I had access to was in the Pontiac 6000 sedan we purchased in the late 1980s and it relied on the Motorola system. The sound quality on the AM stereo stations (mainly KOOL Gold 960 and sometimes on KCWW 1580) was actually pretty good though it heavily accentuated the treble over the bass. But, as noted with FM, you had to have a clear reception of the AM signal before the AM stereo would kick in.