Fixed seats are an excellent track upgrade with some significant compromises. First, long road trips can become brutal and painful. Second, your back seats are only really accessible from the trunk. Third, more of a PIA to get in and out of. Gets old pretty quickly when DDing the car.
I tried some Recaro Profi SPG replicas as a test and they were awesome at the track, brutal for daily driving. I now have some Sabelt Steel Tube frame seats which are better and tolerable but need more lower padding...easily fixed. Still, its barely tolerable for a 3hr road trip. Again, awesome on the track. Its much easier to stay ahead of the car and feeling what is going on.
If your concern is being belted in, I'd just get the Schroth Ralleye 4 harnesses and be done with it. Seats take quite a bit more commitment and won't be worth it for everyone. The min I would recommend are the Ralleye 4 harneses...I've had the OEM belt lose its tension midcorner...made for an interesting moment.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red