I doubt the cat will have any issues with a longer tunnel, as noted by mrcharly, cats generally like narrow confined gaps ! They are also a lot skinnier than the fur can make them appear. When I gave Talisker a bath, and his fur was slick against his skin, he was about half the diameter I expected.
Zev took forever to learn how a cat flap worked, I had it propped open for weeks (when I was in the house) before she realised she could shove her way through.
Kai was happily using it, so you think she would have copied his example.
I put mine through a solid wall, my house predates common usage of cavity walls, and it's not terribly long, but still longer than a conventional door fitted catflap. This hasn't worried any of the cats, other than Zev, but I think any cat flap was going to confuse her.