Whether stock will be better will depend on the model of Sparco and how they are mounted. Since they do not have back rake adjustment, then I am guessing they are one of the race type buckets. If they are bottom mounts, you are limited to changing the actual mounts (i.e., removing the sliders) or changing to thinner padding.
If they are side mounts, you probably have some adjustment at the rear to lower just the rear part, which will give you more headroom. You could also try different side mounts or drill new holes in the side mount to get the seat close to floor. I am 6'3" with a long torso, and I am tight with the stock seats. I fit much better with the seat cushion removed and a thinner cushion in its place.
Most of the "race" type seats are about as thin on the bottom as you can get, but they can still be mounted poorly to take away that head room.