I can vouch for all of Asus 990FX series motherboards and IOMMU capability. I've personally tested the M5A99FX (?) and the Crosshair V Formula-Z. I'm currently using directpath with the Formula-Z to pass a 3ware 9750 into a FreeBSD VM.
This is going to be a long initial post, but please bear with me and hopefully someone will be able to give me some help. About three weeks ago I purchased the following setup:
Antec Signature 650W
Intel Q6600
Asus P5Q Deluxe
Visiontek ATI Radeon HD4870
2x2GB Corsair Dominator PC2-8500...