Surely it's a personal preference? I find myself wishing for a large range cassette on the roadie with the hills and mountains around here. That said a 40t is seriously big seeing as mountain bikes only just started using 40t and above when the 1x drivetrain came about. I imagine you're running a 38/54 at the front? I would be happy with a 32 at most at the back, but i'm naturally a bit of a spinner and ride at a high cadence.