I'm wondering about adjustable strut rods, those can change the caster, but doesn't that make the LCA pivot at an angle and put a strain on the LCA bushings?
You want to use the adjustable strut to fine tune the arc of the lower control arm which may allow an additional degree or two of caster. Because they move in conflicting arcs, there is always binding between the strut and lower control arm, especially as their range of travel increases. However, you do not want to use an adjustable strut to dial in all your caster because, as you point out, it can create additional bind and that bind could potentially occur within the usable range of motion instead of at the extremes. Caster is better accomplished by moving the the upper arm.
Thanks for the shout out CP. Great idea for a pinned note! FWIW, the '05 Mustang GT specs are -.75* camber, +7.1* caster ( which you wont achieve with a mopar, but take as much as they can get while offsetting the camber spec) and .1* to .2* toe in , which on a 26" diamter tire, is right about 1/16" total toe in. I know there are some shops out there that will only use the specs that are in their computer and they will not tackle a customer provided number. Using Mustang specs give them a late model input and a target to shoot for.
The reason for such high caster angles is that you want a caster angle that will offset the built in spindle (or kingpin for old school dudes) angles when you turn the car and the body rolls over the suspension. This offset in angles creates a greater dynamic change in the suspension as it moves which allows the tire to remain more upright while the body rolls. The more upright the tire, the greater the grip it can produce. The greater the dynamic change, the less static change you have to use thus meaning the more postiive caster you have, the less negative camber you have to install initially. Most stock mopars have spindle angles between 6-7*
High caster angles also assist with return to center of the steering. The increased caster requires greater effort, which also helps offset some of the overassist feeling that a stock mopar steering system will have. If you have a manual steering box, you may want to be more careful with using high caster because of this extra effort. Finally, high caster helps with vehicle stability at speed so your car will feel more solid on the highway and going over road irregularities.