Mike,
As with most things concerning a dual street/track car, you'll be dealing with trade-offs here. The basic problem is that the better you get the suspension for the track/auto-x, the less comfortable and practical it will be for the street. You are the only one who can make the decision of how far down that continuum you are willing to go. Of course, the other variable is cost. That being said, here's what I think.
The first improvement you can make is roll bars. The 996 likes a lot of front roll stiffness, so upgrading to a thicker, adjustable bar should help no matter what else you do. The GT2/GT3 bars are a good and they are adjustable, although there are some aftermarket bars out there that are similar and may cost less. Having adjustable bars also allows you to do some tuning at the track without an alignment system.
Next you can work on the shock/spring combination. Any replacement/upgrades need to be done in matched sets. By matched, I mean where someone has done some testing to ensure a particular shock works well with a particular spring rate. This is a complicated area with lots of variables, so this is one place where working with an accomplished suspension tuner can pay big dividends.
In this area there are three strata above where you are with the M030:
Adjustable street systems like the PSS-9 or H&R (approx. $2500 installed)
Double adjustable, remote reservoir systems from ProTrac, JRZ, Moton, and others (approx. $5000 installed and up)
Triple adjustable, remote reservoir systems from the same vendors (approx. $7000 installed and up)
The street systems are obviously the cheapest and may be the most compliant for street use. They do offer some adjustability for rebound/compression but generally not to as fine a degree as the next two levels. I don't auto-x but some of the people I know who track their cars in DE are not completely happy with the PSS-9 set-up for the track, feeling it is too soft.
The double adjustable set-up may be the ultimate street-track set-up but it is significantly more expensive. You also have to find some place to mount the remote reservoirs, which may mean some cutting or drilling, although nothing major. They are harder to install and maintain, requiring a nitrogen canister to recharge them and a special tool to check them. Not a big deal but some extra expense involved. For street comfort, you can probably go as stiff as 400# front and 650# 7" rear springs, which is still very soft for the track but will feel very stiff on the street. I ran 650# front and 800# springs rear and most people (including my wife) considered the car to be too uncomfortable to ride in. Of course, it didn't bother me at all :lol: Just as a comparison, a Cup car runs 1200# front and 1300+# in the rear and a GT# RS runs several hundred pounds more than that.
The one nice thing about this set-up is that you can go full soft on the struts for street duty and with a couple minor adjustment procedures and a shot of nitrogen, stiffen them up for the track. I know someone with an 03 996 that is running the JRZs and he swears his car rides better now than it did with the stock suspension. I haven't ridden in the car so I don't know if I believe him, but the point is you don't necessarily have to sacrifice ride all of the ride quality to get good handling.
The triple adjustables are for pro racers who know what they are doing when it comes to fine suspension tuning and I think the price performance just isn't there for an amatuer-driven street/track car.
BTW, don't get sucked into strut braces, they are a waste of money IMHO. However, a bolt-in cage that uses the strut tower mounts will offer a significant increase in stiffness and a welded cage will provide substantially more. There are also some things you can do to eliminate rubber from your suspension that will help with turn-in and set, but that's a topic for another post.
My final piece of advise is not to try to do this as a DIY. Find a good shop that has experience with suspension tuning for the track on 996s and work with them. You want to get educated and know what you are asking for but a competent shop can save you a lot of expensive mistakes.
Karl