***UPDATE*** I wound up purchasing the Raggtopp kit and, after waiting for a sunny, warm day, decided to use it. Dusted off the top, rinsed with clear water from the hose, then applied the cleaner to the top. Used their horsehair brush to gently agitate away the dirt, then rinsed away the soap. The top looked great, but had to let dry in the direct sunlight for a couple of hours before applying the treatment/water repellant spray. Did so in four coats, pulling the car into the garage, applying the spray, then pulling back out to let dry in the sun for 20 minutes between each coat. The top has now "cured" in the garage for the requisite 48-hours and should be ready to go! Looks a bit darker than before the work was done (a good thing!), and I'm sure that the water will bead like crazy once hit with the hose. All-in-all, Raggtopp was easy to use, and simply took a little patience and an afternoon's worth of time to apply correctly.
Give it a try; Raggtopp.com , and buy the kit with the cleaner and protectant. The brush is sold separately.
Will update when I get it wet, though my 987 is not a daily driver and doesn't see much in the way of rain or other nasty weather.