By "stock uppers," I assume you mean Rear Upper Control Arms? If so, I should mention that I have UPR adjustable upper and lower rear control arms.
Stock, rubber bushed, stamped channel arms. These are designed to twist and grow as the suspension articulates. But during the process, the bushings and arms can only twist so far; when they can't, the result is bind. When bind occurs it essentially creates a rigid suspension and all traction is lost typically causing unexpected snap over-steer. When you replace the uppers with arms that will not allow the growth and twist of the stock arms, you create more bind than with the stock stuff = not good for handling.
It sounds like even though your original setup (300-lb coil overs up front with conventional springs in rear) was not rigid enough for your personal use, they were still adequate for both street and strip and with the conventional springs in the rear, the setup will still be good on the strip. Is that all safe to say?
Rigid is not what you want for handling; you want compliance so the suspension can react to the road conditions and driver input without skipping over stuff. In drag racing you want weight transfer to the rear for traction for a very brief few seconds. Not at all interested in how the weight moves from side to side and front to rear in a AX car in 60 seconds, or a OT car for 20 minutes. When we move either car to the street, ride comfort is generally compromised to some extent. The question is, what are we willing to tolerate?
The other setup I have been researching is Bilstein dampers with #300 coilovers in the front and MM Road & Track springs in the rear. At the same time, I plan to get the MM XL subframe connectors, too. I'm looking at this setup as building my own version of the MM road and track package. From what you are saying and from what I'm reading on MM FAQs, this would give a good all-around setup and would still be greatly improved over my current setup.
I'm most familiar with my own iterations. The worst was using the stock upper arms and some performance springs. The extra wheel rate added by the uppers nearly made the car not enjoyable to drive; buckboard like. And frankly, the handling sucked. The most improvement in ride quality and handling came with the addition of the PM3L. Cheap and effective; transformed the car. But it has some down-sides. Lots of gear noise transferred to the cabin. And you need to watch the upper chassis side mount. There has been damage reported from guys running lots of HP/TQ and sticky tires. I was fortunate to not suffer any damage during the three season I ran my PM3L.
I am kind of torn between the Bilstein non-adjustable dampers and the Koni SA dampers. The adjust-ability seems nice to have if I want to take long trip in the fox I can soften the dampers and If i go to the track on the weekend I can stiffen it. But on the other hand, I think I would rather focus on my driving ability rather than the rate of the damper. What are your thoughts on this?
I put some Koni SAs on my spouse' s 01 Prelude. Also converted to adjustable COs. While the concept of adjustability is attractive,
once we got the dampers set for AX duty, we left them alone. This is because on that car, the lower shock settings caused the car to hit the bump-stops way too often; very uncomfortable. And ride height was not nearly as low as the Mustang. Frankly, the SAs leave a lot off
the table. Not nearly as impressed with the Koni SAs as I am with the HD Bilsteins.