I used to use Deeluxe's AF600 boot (my weight is about 150lbs). With the stock mechanism on 4 on the front on 3 on the rear. Had 2 problems with the boot. 1) too soft when it was very warm outside 2) when I would make my first runs on a mountain my foot would ache super bad as I found it was the lower shell stiffness was just too low and the boot would actually bend quite a bit in the binding (TD1,2,3).
Solution: T700 boot. MUCH stiffer boot. Never once have I had a problem with foot pain and it's been at least 5 years. But a new problem occurred immediately. My AF600 actually had a failure in March one year in Aspen so I drove over to Bomber in Silverthorne and bought the T700's. Fin didnt have the BTS's in stock. Took the boots back to Aspen and even though it was 60F at the base and 45F at the summit I basically could not bend the boot on toeside with the stock mechanism locked on 4/3 front/rear. Too stiff. When the BTS's came I put all blue's in the front boot and yellow (soft) in the rear boot toeside. Exact flex I want and the shell doesnt deform. Also, the failure of the first boot will not happen (the AF600 is so soft when it's warm out that the hinge pivot rivot joint deformed enough to fail the rivot). BTS also has a more even flex when in warm AND cold conditions as it doesnt rely on flexing the plastic so much...the spring does the work of the flex (mostly). Even with the stiffest springs (red) the boot will be softer than the stock mechanism.
I have alot of forward lean in the rear boot and it still works for split boarding too :). Made it to the top of A-basin in my AF700's with Fin in January with no issues with the boot.