Most aftermarket systems have the atac removed. It's commonly regarded as the NS's Achilles heel, due to the compromised shorted lower pipes. These won't do much for the midrange, but will improve top end a lot.

Another thing to watch out for is spares, they're getting on now, and crank parts, pistons etc are very hard to get hold of. If you want it, it's not the sort of thing you'd thrash around long term. Treat it well. Fun bikes, not much more top end than a good NSR, but way more midrange. Two up was never an issue.

I've just finished a rebuild on one of these.