To get quicker response I think you should change the front fins first to the 13cm Drake ones. That will speed up the response time an also soften up the ride. But smaller rear fins will also help. So if you want to get max looseness from the board, My recommendation is that you get a set of Drake 13cm front fins and then try to fins a small set of (symmetric) back fins around 10cm or so. Since fsc fins are so common it should not be difficult to find a pair. You will loose surprisingly little in early planing when going for smaller fins, at least as long as you're on 5.0 or smaller.
But in the end, it is impossible to get the same extreme quick reactions from any quad system as you can get from a twin. On the other hand the quads offer more drice and speed through the turns.