I think you flex looks adequate for more wheeling. Because the trucks are so light in the back it's difficult to get a lot of flex in the springs without going so soft that you lose stability. Shocks can limit the travel in the same way that bump stops and limiting straps do, but they don't change how the spring flexes. I have never run Chevy springs but would anticipate that the leaves are fairly thick to handle a full size load making them less flexible than a comparable mini truck spring.
Alcan springs are the best way to go for both control and flex, but if you went with an All Pro rear spring I think you'd get more flex as well.
Do you have any pictures of the double shackles in full compression and extension? It's difficult to see them in the pictures you have posted.
