If you know the IBU of the kit then yes, you should be able to just add it in. It looks like the toffee malt is a caramel/crystal malt? That's a heck of a lot of crystal then at like 30% of your grainbill, it's likely going to be sweet. For an IPL I think you're kind of light on the late hop additions and a little low on gravity. I know it's sort of a style in evolution but the ones I'm thinking of are pretty big hoppy lagers, as you said essentially an IPA with lager yeast. If I were aiming for an IPL I would up the late hop additions, up the gravity a bit, and greatly decrease or eliminate the crystal malt. If you're going with a lager yeast I'd also ferment it at proper lager temps, I think one of the attractions of the style is the clean lager yeast to showcase the hops. Just my opinions, I have yet to actually brew one though it's on my list and I brew a lot of IPA's. Are you dry hopping? The timing of that is one of the things I'm trying to figure out, I think I'll probably do it in the keg after lagering.