From my experience, a 1.5" change in plenum height on my hemi moved the power enough to change my 60' ET by over half a tenth. HUGE. Runners were also slightly larger, but not much. Both were single dominator cast manifolds. I test on the track, not the dyno.
Going from 7" to 5" should do what you want. If it isn't enough, look into a Brodix turtle or extend the runners.