The amount of slip, or rather the amount of reverse torque needed to initiate slip, is determined by the steepness of the ramps and the spring force of the "spider" washer. The majority of them are not adjustable. I am not sure about the pukka race items though.
Don't have one myself but on Monday was talking to Frankie about Silverstone over the weekend, particularly where I was watching at Bridge & Priory. He is using a slipper but said that at his race speed there is still too much engine breaking on the entry to some corners forcing him to get back on the gas a bit earlier than he would like.
I believe that with the six post slipper, adjustment can be made by way of spring tension as well. Don't ask me how, I think I read it on one of the ducati specialist sites.
The six springs have nothing to do with the adjustment of slipper torque. They determine the amount of pressure on the clutch plate. I guess if you take two springs out, for example, it may affect the slipper torque slightly. Certainly the lever force would be less, but the clutch might slip when you don't want it.
When the reverse torque makes the clutch slip is determined by the height or size/amount of plates in the clutch. This is why you find race teams trying a variety of solid plate thicknesses to attempt to adjust when the clutch slips.