No one knows until it is released and being examined. Apple never publishes any information about these stuff. We don't even know whether its CPU is Coretx A9 or A15 (the latter is more likely if it wants to remain competitive in terms of spec for another year).
No, it's definitely not A15.
Based on technology advancements, and Apple's suppliers, they haven't lined up advancements necessary for the A15. Benchmarks from this morning corroborate this.
Apps will not be released on the App store that don't perform well, because they are being designed with iPad2/3 in mind. You would be fine.
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who would keep an ipad for 3-5 years?

who would keep any electronic device for 3-5 years?
My home PC is a Core2/4GB system I built in 2006, ~5.5 years old now, still doing all I need from it and more. It's completely feasible. Most family and college buddies, all hardware and software engineers are mostly using Core2's, a few i5's. Core2's on desks around office. There's hardly a justification for more outside of synthetic benchmarks.
iPhone 4, I can't fathom having any reason to 'upgrade' it. CPU is powerful, GPU is powerful, retina IPS is brilliant. For web, maps, social networks, sms, it's already overkill. It's pretty much as big as a phone can get and still be easily used one handed. There has yet to be an iPhone 5/6 rumor that sounds luring, most sound worse.
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Two problems with a 45nm A5X / larger battery:
1) Weight / size of the iPad (self-explanatory)
2) Likely shortened battery lifetime. If the A5X is 45nm, it will be big (~160 mm^2) and will likely get quite hot under load. Hot iPad innards = significantly shortened Li-Ion battery life.
Additionally (and this is my personal bias), the 45nm process is just old. I bought a 40nm Radeon 5870 in 2009. Now it's 2012, and buying a mobile device (where power usage/battery life are paramount) built on a 3-year-old process just seems ridiculous. Die shrinks are coming about slower and slower nowadays (it's taken quite awhile for the 40nm -> 28nm transition; Intel is supposedly having significant problems with its 22nm process), so if you're concerned about product longevity, I'm thinking the optimal time to buy a mobile device is on the leading edge of one of these die shrinks.
Have you noticed the new A5X has a nice fancy metal heatspreader on it? Rather than the old ceramic packaging. Likely to help bond it thermally to the metal rear chassis. Yea... there's a reason for that ;-)