Just my opinion, but that's kind of the nature of diaphragm pressure plates. Unlike regular pressure plates, which have a linear slope of pressure, the belleville springs go through varying pressures as they travel through their arc, which is normal (I know I'm stating the obvious here). The problem with that is when aggressive materials don't jive well when the belleville spring hits the point of 'popping' past its high resistance point. When an aggressive material starts to contact at just that point, it makes for an aggressive engagement/disengagement. You can try and manipulate the point where the spring 'overcenter' and clutch plate interact, but you don't have much area to play with, and it's often just a waste of time. The easing over time is just the belleville spring losing some of its 'spring', which makes for a bit of a smoother feel. Personally, I don't want to wait years for that to happen.
I know all modern cars use diaphragm pressure plates now, but a factory system is not the same as an aftermarket that has to deal with tons of variables. Stock systems are designed with ALL parameters taken into account, and auto makers have a fair amount of money to invest in testing...to say the least.
I never liked the feel of diaphragm clutches. The sudden easing of pedal pressure as it travels over-center on the spring just makes for a weird feel on the pedal. You get used to it after a while, but I see that as a compromise, not something desired. I've used them with good success on a couple occasions, but never liked the feel. I've also destroyed a couple of them on performance situations, and will never go back to them for anything performance oriented.