Well, I'll add my $0.02 since you guys are going through the same issues that came up over a year ago. I'm running a 4.5/6.5 drop on my 4wd and it's currently built for top speed and handling so I should be able to help you out a bit in that department. I run the front GF springs because that's the only drop spring for the 4wd available but I am NOT running GF rear springs, I'm running a cut down set of the McGaughy's 4" drop springs. The springs need to be matched to a set of shocks and that's the biggest problem with them, they are poorly matched to both the stock and Belltech shocks. Upgrading to the adjustable QA1 shocks will give you a night and day difference and you will be amazed at the performance difference. I did not like the Belltech springs compared to the McG's ones at this height and since the GF springs are not progressive you drastically change their ride characteristics when the springs are cut, especially when compared to the McG's springs and that's why I kept my McG's springs and sold all the others.
As for the speed shake, have a look at your line of thrust, pinion angle, and rear control arm angles. A full set of adjustable control arms and pan hard bar will get the line of thrust back inline with the body and will allow you to reset the wheelbase and pinion angle. If properly aligned the length of the panhard bar is more than sufficient to give you a good ride and the difference between our panhard bar and a watts link is less than 1/16" lateral deflection at full travel and I'm sorry but until you have had a perfectly solid ride in your truck at 170mph you won't learn that the panhard bar isn't the issue, but it does need to be aligned properly and the McGaughy's or Belltech brackets don't do that. You can install the bracket but then get under the truck and drill the frame bracket to get the panhardbar horizontal at ride height and then you'll be much happier; the adjustable panhard bar will align the axle with the trans and you'll be in much better shape for both the vertical and lateral pinion angles.
Nobody has put together a package that's built for performance which is why I had to put my own together using parts from pretty much every kit out there. Front: 2" drop McG's spindles, 2" GF springs, JTSVP bumpsteer kit, w/ Kore high angle CV joints and soon will be RS9000XL adjustable shocks. Rear: McG's 4" springs cut down 1" (4wd sit higher in the rear), Spohn adjustable delsphere and delrin bushing control arms, hellwig sway bar, QA1 dual adjustable shocks, and most importantly a complete axle custom control arm and sway bar mounts for proper alignment. I also have a set of the Durango A1000 bags in the shortened springs.
I attempted an air spring kit for a few months but as you found out, there isn't enough compression in the kit to fit properly and I was running on the spring's internal bumpstop when I went through even a 1" suspension compression. I've been on a bronco that had less rear end kick than my truck did hitting a bump at 120km/h. A new frame mount would be required to get the required clearance to do a set of bags on the lowered trucks.