Update: I've just installed my harness today (2 holes + backing plates from revolution racegear). Feels really nice and goes well with my Recaro's too
As for the hard top thing, here's what I found from miata.net :
Q) Why would someone get a hard top?
There are a number of reasons. Here are some:
* The Spec Miata racing series requires Original Equipment (OE) hard tops or no top at all.
* The hard top increases chassis rigidity and reduces aerodynamic drag for better performance.
From the Spec Miata site:
e. Convertible tops and attaching hardware shall be completely removed. Cars may compete with the Mazda factory detachable hard top in place (latches shall be replaced with positive fasteners), but it is not mandatory. When no top is used, driver shall wear arm restraints, and the cage will meet the helmet clearance rule.
So you can either race with the hardtop or no top at all with a cage, but the soft top is no go. I wonder why? So if the hard top does nothing, why is this rule imposed?
Reference:
http://www.miata.net/garage/hardtop/http://www.specmiata.com/specifications.htm (Body/Structure section e)