I have slightly firmer springs and shocks but ive also been in one with a similar set up to mine and an arb and the feeling into and out of corners were night and day.
If you fitted slightly firmer again springs it would have fundamentally the same effect as adding the ARB.
So your firmer springs are like adding the ARB to standard springs, if you have your springs and add an ARB it will be similar to if you just went harder again on the springs from where you are now.
All its doing ultimately is increasing rear end roll resistance, firmer springs will do very similar.
Like I said though, on the road, ARB is more pleasant as it isnt active for things like speed bumps where both wheels move together anyway.
Dont misunderstand me, I think all cars should have a decent bladed ARB setup to allow easy fine tuning, im just saying its not the only way to increase roll resistance.