The Google Camera app found on the Pixel devices was ported with working HDR+ mode earlier this month to pretty much any device that ran on a Snapdragon 820/821 or 835 chip to install the APK. The problem was that although the functionality reflected that found on the Pixel devices, the experience was somewhat lacking.
Through the various updates that the Google Camera Port received the various bugs that existed in the initial version were fixed. In the latest update to the APK, Zero Shutter Lag has been introduced for automatic HDR+ mode.
The Google Camera port seems to make use of the Hexagon 680 DSP in these Snapdragon SoCs, so no equivalent Exynos or Kirin SoC will be capable of using this port. The addition of Zero Shutter Lag enables some of the devices with capable hardware but not so great software increase the quality of images it outputs.
The update to the Google Camera port comes courtesy of B-S-G over at XDA Developers who has added ZSL, but only for automatic HDR+.
Automatic HDR+ uses a lighter algorithm than forcing it, and the lighter algorithm uses fewer photos to stitch together. This is why it can use ZSL, as otherwise there would not be enough time to take enough photos. If you use forced HDR+, then you cannot benefit from ZSL.
You can download the latest Google Camera v4.4 Mod with HDR+ APK optimized for Snapdragon 820/821 devices right here.