The `slimmedGenJets` are from `ak5GenJets`, but we also have `ak8GenJets`. I would naively thinking, this could be more appropriate here. One would end up with something like `slimmedAk5GenJets` and  `slimmedAk8GenJets` as possible inputs.
BTW: `addJetCollection` offers the parameter `genJetCollection` (`cms.InputTag`, which defaults to `ak5GenJets`) exactly for that manipulation.