Stiffer means more stability with increased corner loads. I'm sorry you don't get that. And how do you know the exact spring rates and damper force of BMW's?
Did you measure them? Considering most of the recent M cars are heavier than the Z, the spring rates are unquestionably greater. So clearly you have no accurate bases here.
You glossed over this >
Which is as much of an explanation needed on why the Nismo suspension can handle better tires, which is what i said several times now. A tire with a higher level of grip will create a greater level of corner loads during turns, acceleration and braking. A softer suspension would be too loose and won't allow the tire to work properly. Too much roll, dive and squat is increased weight transfer, which is not good for tire traction and balance across all the points of contact.
I'm not trying to get into this debate as this is not a discussion on suspension setups. I've made these debates already in the suspension and track setup sections. My position is already argued on this matter, your lack of understanding on these dynamics should be shifted to those threads.
Anyone here that has used different tires will tell you, one tire may feel better or handle better with a certain suspension setup and not on something stiffer or softer. Firmer works better with high corner loads, softer feels better with low loads. This is not a guide on vehicle dynamics. If you want further explanations on this, PM me or go read a book on vehicle dynamics. I have several suggestions. If you don't like reading, modify your car accordingly and go test it out for yourself at your local track or Canyon like i have and many others here. AND lastly if you've already done either-or, then i don't know why i even have to explain this any further.
You are having a debate about "comfort" when my point was about actual handling physics. They are not mutual. If you want to go faster, you need greater stability. You get this from firmer chassis control and better tires. This is what the Nismo offers out of the box compared to the sport/touring package, apart from the tires. Thus my suggestion of only needing to upgrade tires.