To me it looks like the planted bases sit higher off the floor than they need to (look at the rear mounting point, it is not flush with the base, it is raised up looks like around 1+ cm). I've got both of my Recaro Profi XL seats directly floor mounted using the Brey Krause 9044 floor mount with integrated sub bar:
R-9044 Seat Floor Mount Adapter with Integrated Sub Strap Mount - R-907X Series Seat Mounting Brackets, 996/997/986/987 Porsches)
I got both of mine used at different times for around half price or so.
Then they probably have a side mount for Sparco as well (I am running the R-9075/6) for mine, even though that side mount works with the OEM sliders, it also works for the floor mount. I like it because it has an integrated seat belt mounting point.
One of these days I am going to see if I can get the Recaro XL Halo seat in there...