I have a 6+ and I can't answer that question. I'll probably sit this one out and see if the 7s is more compelling (or if my 6+ is less compelling by then).
I can't blame Apple for it. I think they've reached feature saturation. The stuff is just Good Enough now.
iOS 9 seems to have fixed some of it, but my 6+ still lags on occasion - the 6s+ seems to have fixed it and I feel like I should have waited the extra month but my Xperia Z3's screen got smashed and I needed a replacement device and my Android issues saw no end. I will probably upgrade just for the performance improvements alone, sucks that I'm losing a headphone jack but outside of the car I never need to charge + use the headphone port simultaneously so I think I'll survive with the dumb dongle.
You're right, performance is a reason to upgrade. I'm satisfied with mine, but it could be better. Whenever I use my wife's 6s+ I'm mildly shocked at how much smoother and more responsive it is. I don't think it's enough to get me to pay $769+ to upgrade, but it's still a potential reason to.
The headphone thing annoys me enormously, but it probably won't be a factor here.
I can't blame Apple for it. I think they've reached feature saturation. The stuff is just Good Enough now.