So im looking at a set of sparco sprint 5's. the car is being turned into a track/autox car so they will be used with a hard dog bar with 4/5 point harnesses. Will these fit/what rails would i need to have them work?

http://www.sparcousa.com/pseats_comp.asp?id=622

 

yes they fit good with the buddyclub/ebay rails
you will have some room to move it back/forward

 

Is it possible to get lower by using non adjustable rails? and if so which rails would fit our car?

 

It kinda depends on your seat how low you sit- (padding)
But the Buddy Club super low down sliders will get you Less than 1/2 inch of the bottom of the seat sitting directly on the floor.

 

And how would you compare the Sparco Sprint & Buddy Club slider rails combo to the stock seat? how much lower can I achieve?

 

The seat/rail combo in itself is about the same as stock if I recall. I have this combo but replaced my seat brackets with one's that look like this to get a little lower. Unfortunately I can not find who produces these but they made the fit of the seat night/day.

 

If you're looking at those because of price I'd look elsewhere. I had them in my last car and they were "ok". No real complaints other than they weren't very comfortable and the plastic where the belts come through kept falling off. In my current car I have the Kirkey Intermediate Road Race seat. It is hands down way more comfortable. It's aluminum (or ahh-loo-minny-um if you watch Top Gear) so it requires a back brace. I'm sure there is a way to get it in there. I love the thing and wish I would have used it from the get go. I'm not sure what the floor pan of the S2000 looks like but I know a lot of Miata owners set it directly on the floor and just drill and bolt it there.