[ainarssems]

If I go for lots of paste and leave the downpipe dangling or waggling around, some paste will no doubt drop down onto the cats.

It's not just about bits of paste falling in. The stuff that evaporates from paste can contaminate lambda and catalyst surfaces reducing efficiency or stop working altogether if using wrong paste. I avoid using any paste before or shortly after cats. I know any silicone sealant and coolant getting in exhaust will do that. Some exhaust pastes are OK to use but I avoid them just in case.