Jason,
Have you thought about redesigning the rear pod brackets to lower the axle a couple millimeters? On the BMI DB12R I can get the rear tires down to just under 1.7" and still maintain a 3.5mm ride height with the 3.5 IRS spacers. I also have a CEFX C12 car and I can easily run tires down to 1.6" with a 3.5mm ride height. I measured the difference in the vertical axle hole on both the BMI and CEFX rear pods from the bottom plate but I can't remember the exact difference. If I remember correctly, I think it was 1.4mm.
Thanks,
Tom