The Mopar Perf Race Manual suggests setting the alignment (mainly toe-in) at the +1" level to make sure that any rolling resistance in the change of toe-in with the rise in the suspenion under acceleration in minimized. Quite simple. The camber or caster change will not impact rolling resistance as toe-in does.
As far as air getting under the cars at speed, look at the '63 Max Wedge Plymouths and Dodges, for example. The look like they're on their tip toes just sitting there, much less under full acceleration (with their front end being higher due to acceleration forces. No stability problems with them and they get LOTS of air under them at speed due to their normal ride height being what it is.
Chrysler product police vehicles were noted and loved for their high speed stability--meaning over 90 mph, usually. Even at factory spec ride height front and rear.
The famed Chrysler roadability was accented with torsion bars, but the bars are not the full reason. It has to do more with the camber angles in the front suspension as the wheels turn. Basically, the outer wheel gains negative camber with the inner wheel gaining positive camber. Hence, when the car leans in a corner, the body lean puts the wheels in a more vertical relationship to the road for better grip. Other cars did not used to have this camber situation and as a result, in a hard corner they were using the sidewalls of their tires. From what I gathered in an old Chrysler Service Tech manual, that camber situation has been there since the first Hemi V-8s as although the particular manual was printed in the 1960s, the vehicle in the illustrations was a 1952 era vehicle.
When Chrysler went to torsion bars, they also calibrated the suspensions for a firmer ride and less cornering lean to exploit their cars as being better handling vehicles. From what I saw in road tests of that 1957 era, everybody loved the way they handled even if they rode a little stiffer, yet the added firmness resulted in a better high speed ride and handling. Remember Tom McCahill of Mechanix Illustrated having a late 1950s Imperial for his personal car on his cross-country trips back then? Few vehicles of the time had the total blend of Chrysler's go/stop/turn characteristics, part of which might have been due to their law enforcement vehicle involvement back then. There's a series of articles at www.allpar.com by Curtis Redgap that covers these things.
From an article on torsion bar suspensions that appeared in the Walter P. Chrysler Club magazine, one main benefit of torsion bars and why that change coincided with the switch to UniBody construction was that it allowed the suspension forces to be spread out through the body of the car, unlike coil springs that put all of their forces into the engine crossmember. Where the bars are anchored is a very low-stress part of the chassis. Yes, they are more compact and were high-tech at the time, but in the early 1970s the accountants reputedly wanted to switch Chrysler to coil springs as it would save about $30.00/car at that time.
As CAR LIFE mentioned in a 1965 Plymouth Satellite road test, with the stiffer body structure of UniBody to which to anchor the suspension, it allowed the suspension to be better calibrated without having to compensate for body flex. The same orientation has been mentioned in modern vehicles too.
What I might suspect would have more effect on high speed stability than aerodynamics, all things being equal, is that when the front end of the car is lowered in relation to the rear, the caster angle in the front end will decrease to a negative value. Negative caster will ease steering (used in non-power steering situations) whereas positive camber will increase steering effort (used on power steering applications on the same vehicle). Positive caster adds stability to the car too, as mentioned in some Motor Manuals of the 1960s. Positive caster also increases the camber effect I mentioned earlier as being a part of the basic Chrysler front end geometry.
So you start with great front end geometry, add a sway bar to the front that greatly decreases lean in the corners (aided somewhat by rear leaf springs instead of coils), firm up the calibrations, and it results in a highly responsive handling vehicle that is a joy to drive.
As for the strut crossmember/air dam, it'll decrease lift as it will make more air go through the radiator at road speed. Most coil spring front suspensions do not use strut bars so it's just wide open under there behind the front bumper. Even on the cars that GM used strut bars on, the crossmember was designed to not be lower than the front bumper.
One other thing, if the rear end of the vehicle is tilted upward as the front end is lowered, it just might make the fuel tank more vulnerable should someone not see those Imperial brake lights, much less that large car that's looming like a big brick wall in front of them.
The torsion bars will have a particular spring rate (length and diameter) as a basic design criteria, when the bars are preloaded with the adjustment bolt to raise the vehicle, the preload is increasing the load capacity of the bar (within design limits) or rather using more of the ultimate max load capacity of the bar. As the load capacity of the bar is increased relative to the weight supported, the bar will become somewhat stiffer in the process and the vehicle's ride height will increase at that end of the car. Just as a coil spring with too great of a load capacity will act stiffer when a lesser load is supported by it. As the load is removed, that end of the vehicle will rise too (just as with replacing cast iron cylinder heads and intake manifold with aluminum pieces) and the spring will once again act like the same vehicle would act if it had a stiffer spec spring at the earlier (cast iron items) weight.
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