I think if you work on raising the amount of evap in your tank, your temp may go down.
I have no lids, and a 6"ish fan over my sump. with the MH on 12 hours a day in an ambient temp of around 76*ish, my tank doesnt get a hair over 78.5*
This may be the problem to, and solution for, your heat problem. you get no evap beacuse your display tank AND sump have lids.
It's like your tank's way of sweating!
Jesse