The Recaro SPG XL is finally in. It took my 4 days to finally get it in.
My problem with the brackets being too far apart in the rear was not because the side mount brackets were hitting the seat, but because the bolts attaching those brackets to the sliders were hitting the seat. This limited the amount of side-to-side movement on the sliders. My solution was to grind the head of the cap screws down at a 45 degree angle to provide more clearance. It did the trick. It was still tricky to get the seat in.
So the final results are that the Profi SPG XL is definitely lower than the GT3 seat. It is not, however, lower than the stock seat. This is due entirely to seat position. I had the stock seat reclined quite a bit in the car. The SPG XL with the BK mounts is reclined much more than the GT3 seat, but it is still much more upright than where I had the stock seat. In addition, I lost approximately 4" of leg room because the SPG XL is so tall that it hits the rollbar, which prevents it from sliding back all the way. I remove the rollbar padding on the driver side to get a tad bit more room.
If you are looking for a seat for driving on the road that will give you more room, and you never intend to drive it on the track, you may be able to find a seat that gets you lower. I'd personally recommend doing the foamectomy on your stock seats though.
AFAIK, the GT3 and the Profi SPG XL are the only seats with shoulder harness openings high enough for taller drivers. The GT3 seat is definitely NOT designed for tall drivers though. The SPG XL is. It just doesn't work great in a car with rollbars like the Boxster.
I hope this info helps other people. I would have liked to have had this info prior to buying two seat.