What stiffness springs your running front and is going to make a difference here.
Currently running 600sfront, Stock strut and coils, with half a coil removed.
I have tried SS and the base model sway bars and without.
Without the sway and stock springs it's way too soft. The rear spring collapses all the way to the bumpstop and then some. Understeer, oversteer, spin, backwards, it's all over the place. (This was with 300 springs in the front)
SS bar just seems too stiff with one person the car. With two people its better balanced front to rear, but still a little too much oversteer.
Base model bar: what I currently run. With the 600's up front, with one person, the car is easy to balance in the corners with the throttle and can bring the car back if the backend steps out too far. Starting to build confidence in how the car will respond to inputs. But you still have to wait for rear end to basically catch up with front end due to the softer springs, dampening, etc. The sway bar in the corners helps increase the overall spring stiffness.
Note:This is all compensating for not having rear coilovers and proper springs. So use this for what it's worth.
If you have coilovers and a proper spring selection, you shouldn't need a swaybar.