I'm going through a similiar situation right now. I wanted to upgrade the original bucket seats in my '84 Monte. I purchased a set of the Lear Seigler seats from an '84 Buick GN, but the front seats are either too small for me, or I'm too big for them-either way, they're just not comfortable. I'm going to use the Lear rear seat from the GN in the Monte, as it looks way nicer. I also have a pair of front bucket seats from an '89 Firebird GTA, with the power lumbar (the upholstery guy that I'm dealing with calls them the "ET" seats, because of the style of the headrest), but the seats are tan colour, and the Monte interior is blue, and there's ALOT of plastic on the Firebird seats that would have to be re-dyed to match. I found a pair of front bucket seats from a Pontiac Grand Am that was about 2002, according to the seller. The driver's side seat has a manual lumbar adjustment. I tried it in the Monte last week, it fits well, and its pretty comfy, and the seats are almost all fabric, with very little plastic to re-dye, so I think that I'm going with the Grand Am seats in the car. I want to switch the headrests from the solid style that came on these seats, to the style that has the hole in the middle of the headrest, just because it looks sportier to me. Once I get the headrests that I'm looking for, I'm going to get the front and rear seats re-upholstered in blue to match the interior of the car.
The nice thing about working with bucket seats from GM vehicles, is that the bolt pattern where the seat track mounts hasn't changed in at least 30 years ( probably much more than that), so if you can find a bucket seat from a GM vehicle that's comfortable for you, chances are that in most cases, it will fit into the car you have.
I have a Recaro seat that I purchased in the late '80s that I still use today in my daily driver. Its getting worn, but it is still very comfortable, and it supports my bad back very well. With the Recaro, you need an adapter bracket that will mate the Recaro seat to the seat track from the car that you want to install the seat into. When I purchased the Recaro seat, I bought a mounting bracket for a mid '80s Chev S10 Blazer, which is what I had at the time. It allowed me to mount the Recaro seat into just about any other GM vehicle that I've owned since then ('79 and '81 Malibus, Chev Celebrity, several late '80s Cavaliers, '96 Pontiac Trans Sport minivan). I currently have the Recaro in my 2004 Cavalier, but I had to modify the mounting bracket slightly, in order to get the seat into a comfortable position.
I looked at seats from a couple of other vehicles, before I bought the Grand Am seats, but I didn't want to get into a bunch of fabrication, in order to get seats from another brand of vehicle to fit in the Monte. GM does make a variety of comfortable seats, you just have to find the ones that fit you the best.
The BMW seats are probably very comfortable, and they may fit into your car with very little fabrication. I hope they do, and if so, please let us know, as I'm sure others will wish to try for themselves. Like I said, I wanted something that would work, with a minimum of fussing. Good luck with your swap.
Sorry for the long-winded post. Just trying to pass along some knowledge.
