I've never felt the need to try anything like this, you never know though!
If you want to try skinny tyres just mount them on the standard wheels but mount the inside of the tyre further in on the wheel - so there is some wheel showing. That way you don't start changing the geometry with different hubs ect.
Black springs are too soft for the BJ4, especially on the front, you should never have to go any softer than silver fronts and green rears. Try the latest setup I have posted in the setups section
