Springs under is waaaay better than spring over. On a standard axle, the axle forces gain leverage by the distance from the centerline of the tube to the face of the big leaf. Figure 3" diameter tube, so for spring under its 1.5" to centerline plus another 1/2" for the spring pad. 2" total. With spring over, you add in the thickness of the spring pack, so you go from 2" to about 4-5".
Adding portals to spring over makes it more like 9-10". It seems possible with spring under that it might not be so bad since it's swapping sides. +2" for the tube, then -5" for the portal should leave a 3" effective distance, which really should not be that different from a standard axle. But it's 11pm on a Friday, so my math and thoughts might be less than perfect.
