Thanks for your response!
While I agree that this will not have the strength of the original fastener, I disagree that it would be equally effective as not using at all.
The strength difference between a grade 5 and grade 8 is approximately 30%, and while substantial in a one to one comparison, this is one of 8 12mm mounting points (plus 4 additional bolts) and when considering the load this one bolt will be subjected to that 30% difference would be further diminished.
Additionally, this hitch configuration is similar to other hitches that I've installed on full size trucks where the tow rating is 9,000 lbs+ (assuming the GX is limited by the chassis/braking not hitch) thereby further reducing the 30% difference.
As far as difficulty in tightening the bolt without spinning, this will be directly related to the opening I create. If the opening is too larger it will spin and won't work.
The suppliers that I worked with were aware of my specific application (tow hitch rated @ 6,500 lbs) and indicated this insert would satisfy those requirements.
-Eric