Bottom line, you won't see any decent gains on a single cam without considerable investment (high comp pistons, cams, etc.) especially with an NA set up. If you want to help your engine out, you can do the regular I/H/E, but like I said, you won't notice much.

For the single cams, header style won't make any drastic difference, though I would recommend a 4-1 long tube style to maximize your top end. Shorter and/or 4-2-1 headers are generally more beneficial for torque, though I would advise you avoid 2-piece headers, as I've never seen an aftermarket one that didn't have an exhaust leak. DC Sports, Megan Racing, and various other companies sell good headers for the D Series.

I would also recommend changing to a bigger throttle body, and possibly upgrading your intake manifold to improve flow. Nothing drastically big, but Integra throttle bodies tend to be several millimeters larger and more air can make a difference for an NA set up.

Alot of people love the OEM S2000 mufflers, they flow better and are quiet, plus they have an OEM look.

The cheapest thing you can do is keep your engine free of carbon, check your plugs, all of your filters, etc.

I do have alot of D-series parts, the link is in my sig, but theyre for a Y8, not a Z6, so not everything may fit. I'm sure you can figure out whats compatible on Honda-Tech or through google. Hope I was helpful!