Good question, and a subject a lot of people don't understand when talking bolt strength. Strength is a relative term because as strength increases in metals, elasticity decreases. Strong means brittle, not so strong means more-ductile.

The thing is, two fasteners with differing material but the same or similar strength properties may have vastly different modulus of elasticity ratings. In this case, I don't find an elasticity rating for the 12.9 bolts, but a typical "alloy" steel will be more ductile than carbon steel, which is naturally brittle. More than likely, the 12.9 bolt has similar or even more ductile properties compared to grade 8 fasteners, even though the strength properties are greater.

Regardless, a shock bolt mount is a shear application, which means that the main load on the fastener is across the shank as opposed to a tension application (head bolt for example) that has its main load pulling on the fastener head.