Well, the lag and everything isn't just because it only has 1GB of RAM, it's also because the screen is down sampling on the spot. See, the 6 Plus has a point count of 414x736 and this is how imaging is drawn. It is then scaled up (on the 6 Plus) 3 times, rendering at 1242x2208. This is what should be shown on the screen, however it cannot fit because the iPhone 6 Plus has a 1080p display. So, iOS down samples the image from 1242x2208 to 1080x1920. This takes a lot of processing power.
This leads me to believe that either the 6s Plus or 7 Plus will have at least a 1242x2208 display so it isn't as taxing on the GPU. All of these issues though, including RAM usage, are greatly improved with 8.3. Like others have said, 8.3 is what 8.0 should have been from the start. iOS 8.0 left the testing facility too soon. I blame the executives because they want to push out a new numbered release of iOS every year meaning the engineers have a by limited time to work on the iOS.