The Mach1/Bullitt springs are 2 of the very few,
IMHO,
non ricer springs.

That's because they were spec'd by real
design Mechanical Engineers. Oh yea, they also happen to work for Ford and are suspension engineers.
Unlike the BS cr*p that you buy from aftermarket companies. Just about all of that cr*p is done/designed by a "Manufacturing Engineer". And, most of those "Manufacturing Engineers" don't even have a 2 year associates degree. Much less any "real life design experience". And, a good deal of the cr*p you buy at most aftermarket companies is targeted to appeal to the idiot teenager, or car driver. IMHO, the B springs and most of the aftermarket springs are the perfect example. If a Stang is lowered by more than ~1", it
screws up the handling in many ways. Yea, do about
$20+K of mods to the suspension, and you can then lower a Stang more than 1" and have better handling.
One very important factor for springs that will be used on a car that is
ever driving on the street is that the force to make the front suspension to go to full jounce (compression) is the same/similar to stock. Other than a few very stiff handling springs (H&R & MM), the Mach1 and Bullitt springs are the only springs that fit that
very important real life criteria for a car that's driven on real-life streets.
With the above said, I do realize that people lower their car for
looks only. IMHO, if they want to do that, and as long as they know the implications (crappy handling, horrible bumpsteer, horrible tramming, very real possibility of bottoming out the suspension and/or body, etc), then I don't see any problem with it. It's their car, their choice. *I* get very PO'd when people lie to others and say that by lowering a Stang by more that 1", the car
handles better, it's okay for
all street driving, and so on.
So,
*IN GENERAL*,
- The Mach1 springs will drop a Fox body hatch, 5 speed, with AC, with stock heads and radiator, with tires/rims that are the 100% stock circumference, approx 1/2".
- The Bullitt springs will drop the same car approx 3/4".
But, wait, there's more!

Any original spring isolator has compressed over the years. Both the top and bottom. So, a car with orig springs and the orig isolators has dropped over the years
approx ~1/4"-1/2" from where it was when it was
brand new from the factory!
So, if you put in new isolators, especially new poly isolators, you'll likely not see much of a drop from the current fender height.
NOTE: It takes a few weeks for any new isolator to compress a little. That's the "spring sag" that people that don't have a clue about springs, suspensions, or mechanical engineering talk about.
Also, if you have wider that stock tires (such as going to a 17" rim), then depending on the year Fox, you can have fender clearance issues (front and back). The 92 and 93s have the most room. The 4-eyes (like mine) have very little room. Someone did a nice comparison of the Fox fenders over the years and showed how Ford gave more room over the years. Also, the inner liner changed to allow more room.
That's all important if you lower the car.
Also, it's a good idea for any lowered car to get a bumpsteer kit. I suggest the MM kit. Just get the link about parallel to the ground, that's "close enough" and better than it was before you have the bumpsteer kit. Also, depending on the year, you may need a CC plate to get enough adjustment. Again, I suggest MM only! Ford moved suspension mounting points and the strut tower over the years.
So... with all of the above said, *I* wanted to keep my 100% stock, never touched fenders on my
1986 car - no fender lip trimming, no pushing out the fenders, no cutting, no mods at all! However, I also needed 17" rims/tires to clear my M2300K brakes, and I wanted a better handling car.
Therefore, I wanted/
needed a setup that kept very close to the stock fender height of ~27" (again, with the correct circumference rim/tire
combo!).
Oh yea, I also have the heavier auto tranny and a 3-core radiator (hold more water, which equals more nose weight). So, in a case like that, Mach1 springs would result in more than an 1/2" drop. It would be an 1/2"->~3/4" drop.
One thing to remember, Ford used different springs on cars with different trannys, body styles, and engines because each car weighed different.
So, when you read "drops a Stang by X-amount", take that BS statement with a grain of salt.
There are a number of factors involved.
Here's my setup:
Mustang 86 - Mach1 Springs