I don't know if you are tank swapping or not. We were having some enrage issues, and stopped tank swapping to get a bit more dps and got through it (yes, we know tank swapping is needed in hard mode, will deal with that when we get there).
Tank swapping is not needed in hardmode if you have an assassin dps that can taunt during incinerate. I know this because that is how we kill them in hardmode.
To OP-If you have the bosses down to 15%, I wouldn't change too much in your strat. Get a bit more BH gear and download MOX Parser. Take your DPS to the bridge deck and dps the PVE dummy for exactly two minutes to see where everyone is with their dps. Once you know who needs to do more damage, work with them on their spec and rotations.
15% means you are doing a lot of things right. You shouldn't change your entire strat to try and get the last 15%. Change little things. You guys will definitely get them down soon. Just keep working on it and buying that BH gear.