Originally Posted by terrymorse

A 42 cog is outside the spec range for the XTR rear derailleur (40 tooth max).

The chain wrap for that setup is 47 teeth, which also exceeds the chain wrap for that derailleur (45 max).

Many people exceed the spec range on derailleurs. Some say their out-of-spec gears shift just fine, but I have seen too many people struggle with theirs.

I recommend staying within the Shimano spec.