I'm a bit surprised there is this much discussion on this point but, yep, well known fact that if you remove some of the lever arm advantage the wheel has over the spring (go from bucket to coilover) then you need to reduce the spring rate to maintain the same wheel rate
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I have a pretty effective Aeromotions 72" wing and a 2.75-3" splitter and run 20k/9k (rear coilover) with good results. I do run some rake but the ride height is only slightly lower than stock. Lots of front overhang with the splitter and Stillen front end limit me. I don't want to get outside the optimum travel ranges on the suspension anyway.
I saw a huge gain from installing the Trackspec hood vents; more than I expected. Already with a partially boxed 34 row oil cooler, it seems like adding the venting has dropped oil temps by at least 10F.
The next aero mod on the list is adding vertical air dams in front of the wheel arches
-adds pressure on the top edges of the splitter, keeps front tires out of the airstream, reducing drag, and reduces wheel arch pressure -reducing lift and improving brake cooling flow, I hope- and splitter fences to box in the splitter ends to keep the air from rolling off the side.