EDIT: Solution added
Follow the Link from Shmoo (I CHANGED THE RESOLUTION (iPhone 6+) TO 1920x1080 NOT 2208x1242!!!)
Replace the edited Plist with the Original Boot in SafeMode (If BlueScreen appears try again & again & again) When the iDevice is successful Boot, click on Cydia and Delete/ Deinstall this damn f**k Tweak ('Upscale') Edit the Plist again, change Resolution to 2208x1242 and replace it with the File before. Reboot iDevice & Finish!
Looks like it works!
Big Thanks to @shmoo / @DiDA
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NotEriic's post in iPhone 6+ scale was marked as the answer