Change efi or change intakes????

Seems to me after all I have read that most folks have had better results with a single plane with TBI. I know that FiTech claims that theirs doesn't care what manifold is used. Personally if I was in your position I would try the single plane manifold first since you already have the FI unit. A manifold is cheaper than a completely new system and easier to recoupe the cost than the FI if it doesn't improve.