Counting calls to iommu_gas_augment_entry reveals that this change does have an impact. Running:
netperf -H lip3 -t TCP_MAERTS -D 1 -l 30 ; sysctl hw.iommu.gas_augments
repeatedly yields:
Before (counter only):
hw.iommu.gas_augments: 172403
65536 32768 32768 30.00 3415.61
hw.iommu.gas_augments: 236228868
65536 32768 32768 30.00 3597.01
hw.iommu.gas_augments: 492551939
65536 32768 32768 30.00 3573.99
hw.iommu.gas_augments: 761139580
65536 32768 32768 30.00 3314.86
hw.iommu.gas_augments: 1004374839
65536 32768 32768 30.00 3548.69
hw.iommu.gas_augments: 1270171083
After (this patch plus counter):
hw.iommu.gas_augments: 103720
65536 32768 32768 30.00 3731.08
hw.iommu.gas_augments: 172933331
65536 32768 32768 30.00 3438.34
hw.iommu.gas_augments: 339743562
65536 32768 32768 30.00 3643.82
hw.iommu.gas_augments: 515375288
65536 32768 32768 30.00 3632.90
hw.iommu.gas_augments: 689706155
65536 32768 32768 30.00 3632.46
hw.iommu.gas_augments: 864158380