I don't have direct experience with the suspension on this car but from all the other cars I've had, the shocks are a crapshoot as far as how long they will last. I've seen guys with 2.5" drops on stock shocks and the shocks lasted for 2-3 years. Some other guys have ran 1" drops on stock shocks and had them blow and leak in as little at 6 months. This is all on the same car mind you with similar mileage so there is a fair comparison to be made.
I think what you'll find is that as you raise the spring rate and keep the same shocks, it'll be underdampened. I personally feel like the mustang is underdampened to begin with. What you get when your spring rate is too high for your shock valving is it's usually rather smooth on the larger bumps and dips because the spring is getting a strong force that loads it, but the shock is pretty "loose" in comparison. So you get a very fast compression stroke and the rebound is also very quick. On smaller bumps, there is less compression and you feel the little bumps much harsher than you'd expect because it's basically riding the spring more so than the damper. There's no right and wrong about it, it's just personal preference. With a street vehicle, an underdampened car will generally feel more comfortable because you don't get those huge spine shattering bumps.
For drag racing, I can see how running a slightly underdampened set up would be helpful on the launch to help the rear end squat and come back up once you hook. From a road racing standpoint (which is most of my experience, I only went to the strip for shits and giggles), underdampened will usually throw you off because you will lean in on turns but when you exit the apex and straighten out, the car will rebound relatively violently and somewhat unexpectedly in a lot of conditions.
I would honestly go with the Koni dampers if I were you. Their warranty service is superb and the warranty itself is lifetime I believe on all their shocks (at least the Koni Yellows for sure). The adjustability is always nice, especially if you're racing (which I know you do) and it will probably help you dial in a few things down the line that you wouldn't be able to do with the stock shocks. When I had my coilovers, I would always mess with the dampening. On my FWD drive car, I would set the rears really stiff to minimize the weight shift and keep the front tires down. Likewise, the fronts were set on very soft, basically, the reverse of what you would want on a RWD car. Given that my front springs were not that much stiffer than my rears on that car (I ran a 9kg front and an 8kg rear), I'd say it made a fair difference just for drag racing alone.
SD, if you've never had adjustable shocks to mess with, I think you'll like them. The good ones, like konis, actually have a pretty decent range of adjustment where, as the everyday driver of the car, you will notice a difference in the dampening immediately. I say this because your friends will probably not notice a huge difference but if you drive this car a lot and you're familiar with it, the difference is pretty obvious in my experience.
