Brian, I hope its okay to add something to this.
There's a common school of thought that stronger is always better. There's probably no area where that's more prevalent than in bolt selection.
But, I recently read an article in one of the trade magazines which said the SAE (Society of Automotive Engineers) has recently issued a statement advising against the use of metric 12.9 grade (the strongest commonly available metric grade) in any automotive application unless the strength of the 12.9 is an absolute necessity as proven by engineering studies.
This was prompted by all the big 3 manufacturers having experienced breakage of 12.9's which were used in applications where 10.9's were adequate, but the designer chose to spec 12.9 just for what he assumed would be a bit of extra insurance.
The article was just a fairly short blurb that didn't get into the details, but it would be a safe assumption if a stronger bolt is breaking where a weaker one wouldn't, then the particular joint needs the additional elasticity of the slightly weaker bolt in order to survive.
Anyway, I figured this is worth posting for anyone who's working on something with metric fasteners. The 10.9's, which are the closest equivalent to an imperial grade 8, would be most likely to be found in suspension attachments and such, so something the owner might assume to be an upgrade, like putting everything back together with 12.9's to replace the 10.9's, could in reality create a more dangerous situation rather than a safer one.