Most of us go stiffer not softer. I believe Joel had the 6/5 setup and went to 8/6. I would not go to a 5/3 setup. When does the suspension feel stiff? Over washboard sections of road? High speed bumps, low speed bumps? Dropouts or uneven pavement?
I would wager that what you are feeling is not stiffness in the suspension, it is lack of droop travel. When you jack a wheel up, how far does it droop? You probably had 3-4 inches of droop with your old AGX setup. You probably have an inch or less now. Are your Flex dampers adjustable for both height and preload independently? - I believe those are. If yes you may want to raise the ride height (unscrew the damper from the lower mount) and then lower the preload to lower the car back down to get more droop travel.
It is critical to have these types of coilovers set correctly for best performance because the dampers do not have the travel of an OE style strut. If you don't have enough droop travel the entire car falls into ruts and dips in the road instead of an individual wheel. This causes a really rough ride over washboard sections and larger dips or drop offs on rough roads. On smoother roads, you don't notice it at all.
