"yes, redhat did testing, but I'll bet that their testing was of the 'does it blow up' type of thing rather than performance testing"
I wouldn't bet on that. Red Hat has a fairly large filesystem team and performance team and run performance tests routinely, for public benchmarks (useful to convince customers) and otherwise. All the major Ext4 and XFS developers work for large vendors (Google, IBM, Red Hat etc) and I would have expected them to have access to enterprise hardware. XFS is known to scale better on big hardware atleast historically because of its legacy but the gap has reduced considerably in recent kernel versions.