Here, I'll make it easy for you. Here a conversion calculator-
http://www.chapelsteel.com/mpa-psi.html
If you want to enter the numbers yourself, you will see that my origional post stated that a class 8.8 is about equal to a grade 5, which is correct. A class 10.9 is almost identical to a grade 8.
Now that that's sorted out, If you want to say a grade 5 bolt isn't strong enough for this application, that can be
your opinion.
120,000 PSi vs. 150,000PSI, BOTH will hold. And of course people are welcome to find a longer class 10.9 bolt to do this mod if they wish.