if you can figure out how to lift the elbow up more vertically as you pronate, you'll have more topspin.
Vince's description here on Sampras' topspin serve is pretty accurate imo.
http://youtu.be/Mbfl7IV6pQw?t=14m52s
i'd say this is a better technique than hitting a little before pronation to generate topspin. pronation adds racquet head speed and not using it is a bit of waste.