For normal tasks the difference wouldn't be noticeable, but for things like video editing, the M2 is already way faster than the M1 in Davinci Resolve for instance (up to twice as fast because of some additional engines).
M3 with a move to 3nm and possibly to the Arm V9 architecture could be 30% faster or more (efficiency will depend on how high Apple clocks it, M1 could be much more efficient than A12X but Apple clocked it much higher to increase power so in the end their efficiency is similar).
Personally I hope the aspect ratio is not less square than that of the 11" pro, which is wider than the 12.9, as I would be using a 16" in portrait mode too for PDFs and sheet music.
One thing is for sure, I would not expect a 16" iPad to be under $2000 (and the keyboard at $450). However I am afraid that Apple could come with a 14" first (at $1800 for instance with a keyboard at $400), pushing those of us who would love a larger iPad to buy that, and only after a year or 2 release the 16"....