I have a Kirkey 20 deg layback in my 911, its not a street car, but the seat is plenty comfy for me while on track. The dimension you see, 15 inches, is the seat width, you measure your hip width while seated. and buy the appropriate width seat. Allowing for a driving suit etc, if applicable.
Anothe alternative to Kirkey is
Ultrashield
Ive seen these seats at various track events and they seem quite nice.