Developers are deserting the Mac App Store, the iPad is losing customers and Apple is zagging towards a more services led business model. It really needs to consider a unified platform that allows developers to code once and publish to an iPad and Mac app store seamlessly.
I know that the UWP has been less than successful for Microsoft but at least part of that is down to Microsoft's reliance on a single platform to validate its offer to write once, run anywhere.
At the moment there are around 450 million devices running UWP capable operating systems. 420 million of those are running Windows. Those 420m PCs, plus the billion others running older versions of Windows, create their own de facto universal platform: Win32. If you're writing software for profit this is the place to be, the rest (Xbox, Windows Mobile, HoloLens) amounts to little more than a rounding error.
Apple is rather different. There are rather more iPads in use than Macs. The proportions are much more even though. Being able to deliver a piece of software that runs both on a Mac and an iPad shouldn't be too challenging for a developer and should also pay off handsomely.
Right now iPad apps suffer the pricing problems of being seen as a mobile app, with mobile app support and maintenance overheads. Tie the two stores together and the app becomes a piece of software, with the pricing capability of a Mac program.
Performance shouldn't be an issue - performance of the iPad Pro should closely mirror that of the MacBook, at though Apple may have to boost RAM on its tablets for this to work well.
The real benefit for Apple would be a channel towards building a true hybrid device able to compete with the rising tide of 2-in-1 Windows tablets. The only area of the PC/tablet market that is growing.
I wonder if Apple's new iPad Pro 2 will support mouse input when it launches? Because I believe this will be the first indication of Apple's intent to bring Mac and iPad together.