I launch around 5K. I usually feather the gas up and down between 3-5k and then right when I'm about to launch I floor the gas and drop the clutch instantly. I don't slip the clutch at all. I do a quick instant drop and floor the gas. I get consistant 1.78, 60' times with 17'' Prodrives.
I've made over 100 runs down the track using this method so don't be afraid to do it. I've also done dozens and dozens of 5K launches on the street.
I'm also running Turboxs Stage 4 setup with upgraded IC, and race pipe (catless). So I'm making a lot more HP than stock. If I try to slip the stock clutch coming off the line, the clutch over heats (glazes over) and causes it to slip, robbing my car of power.
02 WRX, Stage 4, XS TMIC, Best 1/8 mile 8.08 @ 85 mph