There are a lot of issues in your questions. But just in general I don't see any down side to owning a Zanardi edition. If you can find/afford one, why not? Unless you want to tear it apart and rebuild it as a racer. Then why bother spending the extra bucks.
But autox car comparisons aren't of much good. I have a friend that runs a Ford Escort with a bone stock engine who beats almost everyone including most of the C4s in raw times. Every event he reminds us that it's mostly the driver in this sport and not the car. I run in ASP class because I put the Zanardi suspension on my 94. I've beat a lot of Boxsters, C4s, and a Zanardi Edition. But that doesn't mean I had a better car. (OK, it is better than the Boxster and the C4, but you know what I mean)
The NSX isn't an ideal car for autox to begin with. It's much more of a track car. But you certainly can be successful at autox if you drive well. Unfortunately there are so few NSXs in autox across the country, it's hard to get a baseline on what really good drivers can do with them (you don't see NSXs at the nationals).
With that said, running a Zanardi in AStock should be VERY competetive compared to a stock NSX. With just the Zanardi suspension parts, I'm doing quite well in ASP: 3rd place so far this year.
Go for the Z
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