I've seen the Simpson (rebadged) and the Recaro car seats. They are "cute."
I have used the one Simpson used to rebadge in my NSX. No problems. Fits in the car with no issues. I just move the seat all the way back, recline it so it matches the car seat, and we are all set. My daughter LOVES riding in the NSX with the top off! (She just turned 5)
Just make sure when you secure the car's seatbelt to the carseat, to pull the seatbelt all the way out, then let it retract as tight as possible... the belt will stay locked in position until you release it.