When I head shot with the explosive rounds on it still has the firefly effect, so you can double up on arc explosion goodness! all I've noticed it doing is reducing the damage per bullet, which it says it does anyway.
I'm not sure if the firefly works on all types of enemies though. I don't think it works on harpies/supplicants.
Yeah i've not had it work on the harpies, gatekeeper things (templar/nexus shaped ones and not seen it work on hive wizards yet either.