Lemain, it also doesn't criticise GoPro as having poor performance when inverted. The report points out that the video is of "reduced quality" due to contrast. The term "Poor Performance" seems to be of your making, not a phrase that the AAIB attribute to Go Pro.
The issue is that when (the aircraft) is inverted, the daylight coming in overwhelms the camera compared to the view when upright. The problem comes from the camera being fitted to an aircraft which turns upside down, not the camera being upside down.
Your original point about the camera being able to see the altimeter comes from the JP display, not the Hunter, I think, or did I miss that bit?