Did you tweak the K-member at all? A new one wouldn't hurt anyway.
The MM K-member raises the LCA attachment points 1", so it's more similar to the stock geometry when it is lowered [When you just lower the front, you lower the roll centre a lot more than the centre of mass... which actually produces more roll, and more force on the outer wheel].
It also moves the LCA forward about 3/4", which helps weight distribution. And, it works with the stock LCA's, so it isn't too pricie.
The griggs K-member would be another step up. It requires LCA's and coil-overs, so it'll cost more... but it's got more anti-dive built in, so you don't get as much weight transfer when you're braking [which can really overload the outside front tire in a turn]. Also, as the car rolls to the outside, the tire moves slightly forward, which also reduces it's lateral load. It's all good ;-) [although I'm not really keen on putting the spring load on the strut tower].
I'd wait and see what the Griggs SLA looks like first though... but that'll probably be even pricier.
As for the rear, the spring rate will depend on the setup. SO... you really shouldn't get springs until you get the torque arm and panhard bar (or you'll be buying them again). I'm planning to go to coilovers in the rear.
www.c-f-m.com has a cheap conversion ($40), and then springs will cost about $100... THis will take the spring load off the torque boxes, reduce unspring weight, and be more consistant (the spring mount isn't changing angles all the time).
In the rear, I plan to fabricate some parallel uppers to make a 4 link + panhard, instead of adding a torque arm. I think they're a little pricie (although nothing like the steeda 5-link.. is that gold plated or what?), added weight, and not quite as slick for the corners... adding some brake-squat would be good too...
The mustang is just so front heavy... anything you can do to keep more weight from transferring to the front is certainly good.