The 944 floor pan, like many cars, is mostly just sheet metal. On the early-944 and 924S, the forward seat mounting holes are in a large hollow bar, which will compress and deform when you put a bolt all the way through it for a race seat. And the back two mounting holes are just little raised hollow pedestals on top of sheet metal, which will also compress and deform when you put a bolt all the way through them.

The stock seat only uses small bolts for these, but the nuts are right below the mounting surface (ie not on the very bottom of the car), so it doesn't deform anything and the mounting points serve as reinforcements that spread the load. We compromise that when we mount our race seats. Maybe the fact that we're using much larger bolts and big fender washers will be just as good as stock, but I'm just not confident when I see the mounting points deforming when I tighten the seat down. I added some reinforcements to prevent that from happening, but one of my winter projects will be to weld up something that takes it a bit further.

Here's what was done on Chris' 924S...

http://img.photobucket.com/albums/v64/_scuba_/race1.jpg?t=1253665586

You can't see it in that small picture, but Hank welded those bars up to the car really good... they aren't going anywhere.