As yet another Flex user, I think that Truestripe nonsense holds some weight. 720p looks surprisingly good on it, better than similar devices like the Mega. They have a lot of work left to do on Truestripe and on P-OLED tech, but I hope they can scale it to at least 1080p soon...not really for general definition as for better split screen work area specifically.
OLED tech enables what I call 'BS battery life'. Various reviews stated the Flex could do 14, 18, 20 hours of continuous video. Power consumption is phenomenal as an e-reader, if you read with grey-on-black (e paper is best as black on white, backlit screens better light on dark) you can hit over fifteen hours of reading time...I don't have the endurance to find a practical limit, but it's miles beyond my Tab 3 7.0 LTE (IPS backlit) which can do 10+ reading hours.
The low rez at 720p combined with the P-OLED efficiency has given me up to TEN BROWSING HOURS in Opera on my phone, and everyone knows that browser is a power hog...
Now as to 1440p, rumors about the G3 and etc. LG claims its new *backlit* 1440p 5.5in panel uses a better shaped pixel gate that's more efficient in using the backlight. They were claiming power/brightness equivalence with lower rez screens that have much less wiring and gating in the same area. THAT takes us back to 'just' the raw rez. And THAT takes us even further back to an issue I've seen again and again: "games and browsing".
The top hit to performance is games and browsing. For video and pictures, the dedicated hardware should have near zero performance hit. For texting and typing, the hit is literally zero. For PDFs, Powerpoint and such there will be a large compositing hit that cannot be avoided. The interface? Really depends how much polish they put into it.
GAMING. PCs have wrestled with this for years, and depending on the game engine your loss in FPS could be anywhere from 5% to 70% when you double the pixel count from 1080p to 1440p. A lot of the games for mobile are crappy tho, and should run okay at 1440p even with just Adreno 320. Power consumption will increase a little, but often not by a great amount.
BROWSING. This depends a lot on both the app and the page it is displaying...AND on your device's DPI settings. For example, a 10.1in tablet and a 4.5in phone with the same rez are doing totally different workloads in scaling the page because their DPI preferences are different. I've played around with the N7-2013 and my 7.0 LTE, and the N7 always felt like a slower and sloppier browsing experience even in Chrome. And the battery life...eck. Going from there to browsing at 1440p would push pretty much any phone hardware on the market to its limits and possibly beyond, even a fast desktop with hardware accelerated graphics can struggle when upscaling a page to that rez.
So this ran on too long but yeah, AMOLED over IPS very soon now. And 1440p vs 1080p will have zero impact for some things but probably cripple your browsing and PDF reading performance. 720p is sufficient for nearly everything if the pixels have a good shape and the UI is optimized, but 1080p seems like a good standard to push for mobile devices.
TL;DR, GIVE ME MY 8MPIX LAPTOP SCREEN ON A <$800 LAPTOP YESTERDAY