A new recently released jailbreak tweak called ForceGoodFit, this tweak forces unoptimised apps to run at native resolution rather than in scaled mode. This is great because a lot of popular apps like WhatsApp, Google Maps etc. still haven’t been updated for the iPhone 6 or iPhone 6 Plus.
The tweak is very similar to FullForce, another tweak that forced unoptimised 3.5-inch apps run at full resolution on the 4-inch iPhone 5, iPhone 5s and iPhone 5c.
You can install the tweak for free from Cydia, and enable it in Settings > ForceGoodFit. If you don’t see all the applications in that list, you might also have to install AppList.