Some hdri backgrounds produce "wild" pixels which are very difficult to smooth out and increase a lot sampling requirements for backgrounds. This usually happens with background images with very strong highlights I believe. This issue is also reported here. I would be good if we can find a good and phisically correct solution about this.
HDRI is downloaded from here (Port Syros low res) :
You can also download it from https://drive.google.com/file/d/0B9Xy9o3kawnQMmhxbV9sbTBRQnM/view?usp=sh...
Blender example below. Greetings.