I just ran some speed and quality tests on 4.4.1. High memory, and Max AI was turned on.
I then did overlays using layer difference in Photoshop and discovered that the two images with GPU Off were identical, and the two images with GPU On were identical.
The two with GPU Off had more contrast than both the original and the ones with GPU On.
The histograms for the two that had GPU On showed that their darkest blue values were shifted slightly brighter, but otherwise their shapes matched the original.
There was also less micro detail, and some visible horizontal banding you can see on the rock faces with the ones processed using GPU On. The banding is passable in this case because it looks enough like rock layers (though the slope does not match the strata exactly), but this noticable of an artifact might ruin any other image.
Either version is outstandingly better (sharper / more detail) that simply enlarging the original 4x in Photoshop.
GPU Processing Off
GPU Processing On
If you click on one of the images to make it full size, you can use the left/right arrows to quickly compare and contrast the two.