I wouldn't glyph Havoc unless your raid really, really struggles with Infernals - and I really can't see not enough damage on them being the issue at this stage. The issue will be separating them or dealing with them in combo with other abilities. You can exploit Havoc on cooldown way too much in p1&2 (and 3) to equip that glyph for what will effectively amount to benefit 1 maybe 2 waves of Infernals at the end of the encounter.