So now a hypothetical scenario, The manufacturer decides to put the manifolds on with 3/8 grade 8 bolts now, to cure the problem of 5/16 low grade bolt snapping. The 3/8 bolt cost more but is much stronger, so now that bolt pulls the threads out of the aluminum head instead of breaking. How much is that going to cost in warrenty repairs?
It all come down to the almighty buck and which solution keeps the prophet margin the highest.
The engineers, bean counters, and management, I will bet will all vote to keep using the weaker 5/16 bolt.