Boxerworks.com

I wouldn't go with progressive springs.
With progressive (not the brand specifically, but the type), the springs are wound tight and then wide.
The tight area is "soft" and the wide area is stiffer. With the weight of the fairing, you will take up all of the "soft" with the initial sag. I'd go with a linear rate spring, possibly the BMW OEM Heavy Duty spring used on an RS or RT.

I switched to the BMW heavy duty fork springs on the RT I had, and loved 'em.

Added bonus: I think (if I remember correctly) they were less than 50 bux.