Steve - what are your objectives for this car? If racing, anything near seriously, you'll want to take into consideration the allowances of the class you'll be competing in, and spending some $ on shocks can be a great investment - one of the few modifications, for example, allowed in SCCA stock/street class.
I have experience with Koni sports on the 986. These are a good value for a versatile, but on a budget weekend warrior that maintains stock class eligibility. Not the best setup out there for extreme, no-holds-barred competition by any means, but the external adjustability (of at least the front Koni sports for 986 - rears are unfortunately the style that needs removal to adjust) is a nice upgrade over the oe Porsche shocks and non adjustable bilsteins. You can also have them revalved if needed, or modified (such as to make double adjustable) by outfits like Proparts or Truechoice. This is the cheapest way to get into a double adjustable setup. (The other way to get there is spending probably into the multiple thousands on something like, JRZ or Penskes.)
I hear some mini racers are having good results with FSDs, but they are not externally adjustable so you'd give up the ability to make fine-tuning handling adjustments in between runs or to accommodate changing road or tire conditions. I haven't heard many reviews on 986 FSDs, so not very certain how they are valved. You might try to reach out to Koni technical/service center, as they may know.
Not sure if aftermarket can revalve FSDs, but could ask truechoice, proparts, or Koni tech center. Out of the box, I suspect they will not be totally optimal for your M030 springs and sways. Probably good enough for more of a primary street driven car that you occasionally play with in not very serious competition. They would be the softest ride out of any of the still fairly decently performing options out there, so if that's of large importance to you, could be a good consideration.
One thing I have little doubt of is that you're going to see huge performance benefits upgrading your old and certainly worn out 120k mile shocks - probably in the >1 second range on a typical autox course. Happy to help with the replacement procedure as I recently tackled the job on my 2001 which had about 110k.
Ps - the factory shocks and all aftermarket bilstein options which use the factory sized spring (excluding the coilover/pss9 style offerings) are twin tube. Monotube bilsteins converted to adjustable (using the aftermarket methods I'm familiar with, at least) are not as good as adjustable Koni sports, because the adjustment you'll get affects compression and rebound. Adjustable Konis give you adjustable rebound, which is more critical and important to tune separately from compression damping. I'm not certain whether anyone offers modification of factory twin tube bilsteins to make them adjustable. Probably easier to just switch to Koni sports.
