Tenzo lowering springs...

I dont have any experience with these springs, but if you are just going for the lowered "look" they should be fine. If you are looking to improved the performance of your stang, stick with the name brands like Eibach.
 
I had Tenzo R springs on my 97 Civic EX coupe back in the day. I thought they were a rice only brand, but I guess not anymore. They were a 2" drop and the car handled very well. I used to do pretty well autocrossing. I had them on the car for a good 3 years and they didn't sag at all. Don't know how they'd be for a Stang though. Different ball game there.