Well....I have a (currently) level 47 FIRE/RAD controller (her name is Scarlett Angel) . All I will say is this....after trying different combos....I have settled for 4 DAMAGE and 2 RECHARGES on my imps.WHY YOU ASK??? I AM RADIATION!!! I have PERMA HASTEN AND AM...and LEADERSHIP...and enervating field and radiation aura. I can solo in PI (lvl 50 fake nemesis...no prob). I almost solo'd a lvl 47 AV...the group I was with wasn't doing much damage...I was keeping him held and letting my imps [censored] him. So....to sum it all up.... go with 4 damage 2 recharges only if you are running leadership perma hasten and AM.
And by the way...if you don't believe I can solo +3 fake nemesis...send a tell to Scarlett Angel and I'll show you(she's on JUSTICE)
Later and good luck!