That iMac will surely need a strong CPU and GPU if they expect 4K video editing and 5K monitor. Rather than Apple increasing screen resolution constantly, they should let the other specs catch up first. As for iPad Air, nothing too exciting there really, and I thought iPads previously had 2GB RAM anyways.
The iPad mini display is not sharper. It's higher resolution. Resolution is independent of "sharpness" or PPI. The lower resolution iPhone 6 Plus is actually much "sharper", higher PPI, than the higher resolution iPad mini display because of the size of the displays. Sharpness, or PPI, is a function of the resolution and the size of the display.
iPhone 6 Plus = 400PPI
iPad Mini = 324PPI
While the iphone 6 plus has a higher ppi than the ipad mini retina, it is is also a 5.5 inch device versus a 7.85 inch device. How much closer is one likely to hold a iphone 6 plus to their face? How far away is a person likely to hold an ipad mini retina to their face? Given the relative closeness in ppi(400 vs324) and more disparate screen size( 5.5 inches vs 7.85 inches), I would hazard a guess that to most people, the ipad mini retina may appear to be the sharper of the two devices. It may be fairly irrelevant, though, as at standard usage distance the ability to distinguish individual pixels should have long since dissappeared for most people.