As choco said - "all brands of race/slalom sails are fast these days". I use 2009 6.6m and 5.8m ka koncepts on the 89 falcon - awesome board and sail combo. Depending on your body size and fin selection - 6.6 is a great size.
generally the slow part is the bit between the boom and board
Similar to most computer problems where the issue is usually between the keyboard and the chair.
Fastest slalom sail? Check out race results for local, national and international slalom comps. They usally sail production gear so should give you an indication which brand is the fastest.
The one that you can water start with, more cams-more room to fill the luff pocket with water. Your local dealer should be able to help out here, then it's just down to personal choice of brands. Put some trust in someone who's interest is to keep you sailing.
I had the chance to try out both N.P slalom mkII and Severne overdrive yesterday. I am not sure which sail is faster but I did like the light yet efficient feel of Severne, so I think I'll go with that.