High impact fusions (wizened rebuke and Erentil) should not be able to roll backup plan. Regardless of their lethal range (Let's be honest, it's way too much.), part of countering fusions, especially of that archetype, is sound queues. You hear it charge, so you bait the shot and then punish. Simple up close, annoying when you get mapped at a distance you should have been safe. Also, the disadvantage of high impact fusions is their charge time. It's the longest in their class, as it should be. Backup plans eliminates both of these. Sound queues go out the window. Charge time becomes nonexistent. And the lethal range doesn't change very much at all. Certainly not enough to warrant the advantages of the perk. So in short, high impact fusions should not be able to roll with back plan, as the perk currently eliminates the key disadvantages of the gun with no drawback and eliminates a key aspect in countering the use of fusions, i.e. sound queues.
Back up plans should just get the same treatment as rifled barrel. By that mean is if you equip back up plans in increases spread during the perk being active and decreases over all range.