In terms of space, the boot is definitely smaller. Being a coupe style, the sloping back makes the boot opening smaller. I haven’t tried but I think two full size suitcases would go in but that’s about it. The M5 could fit in two full size suitcases and a pair of cabin bags.
The M8 is longer and wider, so it overhangs normal parking spaces, the M5 would just about fit lengthwise. Interior space is good, the rear seats in the GC has plenty of leg room, the issue is the center console makes the middle seat uncomfortable if that would be an issue.
Some quirks I don’t like, are the front seats aren’t as good as the M5. No idea why they would put worse seats in your flagship model compared to a “lower” model. I use the term loosely as the M5 is a great car and it’s not a sleight on the car itself. The M5 has more adjustment on the back which I really liked and miss, it also would show on the infotainment screen what was being adjusted when you hand was hovering over the seat adjust buttons which the M8 doesn’t. The M8 has inflatable side bolsters though which is nice.
When parking the M5 would switch to an overhead view so you could see if your splitter was getting close to a kerb which was a really nice feature but the M8 doesn’t for some odd reason. That’s about it though so they are pretty small gripes overall.
As a car, I definitely prefer it to the M5, it feels more premium. It feels faster even though in reality it isn’t but that’s probably because your backside is closer to the ground. Most importantly, it’s better looking.