Wildfire capping at 4 stacks is an acknowledged bug (Wildfire not stacking past 4 - #3 by Pygex). Even with the current bugged 4 stack limit you can get it to do a lot of work in the right build. Once it gets fixed I think it will easily be the strongest pick for any Psyker using soulblaze, having a 16 stack rip through a mixed elite horde is going to be quite the spectacle.