I have had the same seats in two Spyders. In the first car, they were mounted flat to the floor. I the second, I added an inch and a half lift on the front, and about 3/8" in the rear. This allowed me to actually adjust my crotch straps on the five-point belts. I'm definitely more comfortable tilted back a bit, my thighs are better supported and I feel more "in" the car.
All of this depends on your torso and leg length though. In both of my cars I did not install sliders, the seats are fixed. But the car is for me and me only. I did also make a set of 4-5 inch pedal extensions for the wife, though! She has yet to use them.
A Speedster has a slightly higher cowl and windshield so you are afforded a bit more room to work with.