The DX10.0/GPU in A8X is under 20nm at 38+mm2. That die area doesn't help because they just mirrored a quad GX6450 but let's ignore that for a second. I'd estimate that it would blow up in die area nearly to 60mm2 under 20nm in theory for an 8 cluster DX11.2 7XT part. Since they're scaling just clusters die area shouldn't scale linearly upwards, but are you sure the bullet for 16 DX11.2 clusters is already "biteable" under 16/14FF? If yes it would mean a ULP SoC GPU roughly as big as the entire Exynos 7420 on estimate
Typically, I'd say that this was unreasonable.
However,
Given the rumors of the iPad "Pro"'s features (stylus, multiple stereo speakers, USB-C port), I think the "Pro" is at least partially aimed at expanding the market of the iPad line.
I think this is correct, and the reason why Apple might conceivably be very ambitious with a hypothetical iPad Pro SoC.
Basically, iPads are Apples idea of the future of personal computing devices, with phones typically taking the tasks of communication/entertainment/photography/video.
They want iPads to cannibalise laptops. And they do, last reported quarter Apple sold 21.5 million iPads, as many iPads as they sell macs, in total, over a year. However, tablets have an unfortunate reputation as being devices primarily for surfing the web, and cheap android tablets have reinforced this strongly. (The other day, I was offered an android tablet for €5 if I filled in a survey. I didn't do it.) The only clear exception to this public impression is Microsofts Surface Pro products. Now these don't sell anywhere near Apples volumes, roughly a million were sold the same quarter Apple sold 21.5, but they are clearly pushing into exactly the segment that Apple wants iPads to (also) occupy. I don't think Apple wants to leave that segment uncontested to Microsoft.
Now, having a larger, higher resolution screen allows more information to be displayed. If they have USB-C, it allows connection to high-resolution external monitors via DisplayPort Alt mode. They can expand memory and storage. Together this makes for a more "professional" device, but it will inevitably be compared to the Surface Pro. Can they allow it to fall short performance wise? If I were a strategist at Apple, I would bite the bullet, and ensure that performance was also at a level that both clearly signalled that a new market was being addressed, and didn't fall short of Broadwell/Skylake in Microsofts offerings, die area be damned.
Of course, I am not actually a strategist at Apple.
God knows what they'll do SoC-wise if they actually release such a tablet.