Cool, sounds like you have a good handle on it. I'm not sure, but I and the few other early adopters may be the reason why IE doesn't list those plates for magride any more. The IE plates are shorter than the stock top mounts. Without going into it too far, most of my year long suspension problems were MSS (non-adjustable) front spring related with the IE plates. The stock springs and IE plates were fine, but the MSS springs with IE plates were too soft and short. Then I bought the MSS adjustable fronts to fix the problem. They have a huge aluminum adjuster paired with a spring that probably has no more than a 2" usable stroke. So instead of getting error codes from the dampers going out of range, now you get a lovely bang when the coil bottoms out. They now call these "track" only front springs and don't recommend them for street use. Why I spent nearly $2000 on a spring kit has me questioning my sanity.
Note: If you are getting suspension codes, don't mess with the height sensors, that isn't the problem. The code is from the damper resistance measurements going out of range and you can only fix that by limiting the stroke, using a higher spring rate, or taller spring. I spent way too long trying to get the MSS kit to work with IE plates, it really wasn't worth it, and required way too much of my time with a VAGCOM researching the damper issues.
I think you are on the right track, I would do the same and try a couple linear springs instead, if I were going to stay with magride. Leave the front springs OEM with IE plates and just play with higher spring rates in the rear. I haven't measured the length/ID of the rear springs, but you could probably find some good linear springs from Swift to play with. You also will want to do rear bushings or at least the trailing arm bushing to stop toe deflection changes. The rear end is vague as hell at turn in otherwise. Change that bushing first before throwing more sway bar or spring at the car so your testing is more consistent.