I was generally able to do one knot higher GPS speed on my old Carve 145 with a 6.5 no cam than I can do on my iSonic 122 with a 7.0 cammed slalom sail!?!
On the other hand the acceleration, pointing, jibing and especially maxed out control of the iSonic is a lot better, so at the end of the day the slalom board would be fastest around a slalom course.
My S-type 115 would beat them both with a few knots on max speed, but loose everything in planing and pointing..
Slalom, speed freeride?? Different boards for different styles but they do overlap a lot as they are basically all high quality and conditions are unpredictable as a design factor.