I figured I'd make this thread considering I haven't seen a lot of feedback in terms of the interior seats and steering wheel on the g80 other than "they're stiff." So here's an opinion from an OG M2 owner:
The regular seats should not be an option on an M3. Although the carbon buckets can be stiff for a casual driver, they provide the exact comfort that you would expect while having serious rigidity of a carbon fiber bucket. Think Porsche but more comfortable, probably because it's adjustable.
In my experience today, the regular seats provide no support. They might be good for casual day-to-day driving, but there's no chance of you staying in place if you try any spirited drives. I tried to deflate the lumbar support and increase the bolstering to its max and I still barely felt any support. I'd say I'm a regular built at 5'10 and 165 lb for reference. The carbon bucket gives you support in all the right places and the bolstering button allows you to choose just how much movement you want in the seat. I could see the comfort of the ride being compromised on long drives but I think it is 100% the right choice for anyone even remotely planning on doing spirited drives. Also, the center piece in the carbon bucket is unnoticeable if that was anyone's concern. I'd also add that for taller owners, the carbon buckets drastically add more leg room for rear-seat passengers seeing as the seat loses about 1/3rd of its overall size.
Moving onto the steering wheel/paddles. It definitely feels more premium. I found 1 complaint with the paddles in both the carbon interior package and the regular. The carbon paddles, although they feel nice and premium, are a little too far in. This is just nitpicking at this point but to clarify, when comparing the f8x paddle shifters, I prefer how the paddles are closer to your actual fingers while the new paddles seem to be more on the end of your finger tips. The non carbon pack paddles are the same as a non M 3 series so I'd say that's a bit of a shame considering the car's almost $90,000 when well-specced.
Overall, the interior is great but I just could not see myself ever wanting the comfort seats unless I strictly planned on using this as a grocery getter.