CrossDMA is the same as CrossFire bridge. Old bridge was ONE PCie 1.1 line, now is ONE from 16 PCIe 3.0 dedicated to sync, function is the same like bridge.

16x PCIe 3.0/8x PCIe 3.0 no performance hit, 16x PCIe 2.0 no performance hit, 8x PCIe 2.0 maybe minimal performance hit, but i think very very small