Thanks Mark for the air filter number. Frank, springs, I am going to use 950R and move the 850's to the front. You're right about high speed aero imbalance. I was getting a strange brake balance feel at the end of the back straight/first 1/3 of brake zone at COTA (+150mph). The car would seam to pitch from rear bias to front bias, but it was delayed as if the rear of the car was overcoming a lower than usual ride height at high speed before it could load the front of the car. It also felt a bit twitchy as I applied the brakes hard but only at that point on the track. Kind of like the toe was going neutral or a bit more positive at the front. But it didn't do that on the front straight brake zone. Maybe the uphill and slightly slower speed made a difference.
At least that's all I could come up with. I tried to re bias towards the front but that just screwed up the rest of the track. My car really likes a 1/2 inch rake with the front lower and if the rear is getting pushed to a rearward rake from aero loading then that would be a possible
I am looking forward to getting back to COTA and doing a back to back spring change from 750F 850R to 850F and 950R.