Ok, yes it is, but I think you aren't really clear on what you are talking about.
If you do not have to abide by any rules like SCCA A-sedan, it is not that good.
Well, i watched the A-sedan race at the run-offs, and I am pretty darn sure that the guy that completely walked away from the rest of the pack was running a 5-link, but I could be wrong.
It still keeps the original quad design in tact, not good.
Well, see, you are a bit confsed here. Yes it does maintain a 4-link configuration (apparently steeda considers a Panhard a 5th link) but not like the stock 4 link. You see, the stock setup has a non-parallel upper link setup, and the steeda is a parallel upper link config, no binding.
The way to really trick out the rear suspension on a Late model Mustang is to remove the binding upper control arms and going with a torque arm system like Griggs or MM offers.
Binding? umm, no binding w/ the 5-link at all. A torque arm setup has more binding than the 5-link, w/ the dissimilar arm lengths and all (not that it is enough to cause problems, but there is certainly more than w/ the 5-link) Also, w/ a torque arm, you have the possibility of having to back out some brake bias to avoid brake hop under heavy braking, and the 5-link has no problems w/ this
This will almost eliminate all binding caused by the crappy 4 link original set up. This basically turns it into a 3 link system and puts the control arms on the same plane as the 3rd link (torque arm).
Yes it does. Though a good, true 3-link or parallel 4-link is even better still, the latter of which can be had in a steeda 5-link. That and if your TA breaks at the front mount, you won't catapult the car (the guy who designed the 5-link had this happen to him)
fwiw, I really hold no ill-will towards a TA/PB setup or MM, but griggs is generally viewed as a better choice in the racing community. Though I don't see how much different the two could be

but then, I'm sure Mr. Bassen has voiced his opinion on the two before I have finished this reply

btw, my apologies for messing w/ your post, it was an accident, though I did not remove any of your text or change it in any way.