I am in the decision process now of which to purchase for my 03' 6.0L.
Here is some info I found, might help others, or not. Either way...

Injen Claims 22HP/21TQ (13HP/12TQ for 5.3L)
Volant Claims 18HP/20TQ
Green Claims 17HP/19TQ
K&N 77HF Claims 14.05HP
K&N GENII Claims 15.17HP
AEM Claims no report
AirAid Claims "Up to 14HP"
AirAid Jr. Kit Claims no report
True Flow Claims 13HP/15TQ (*Just filter and tube, uses factory Air Box*)
Injen Price $347 (cheapest I could find)
Volant Price $285/$305 with oil free powercore filter/$360 with ram air duct
Green Price $250
K&N 77HF Price $320
K&N GEN II Price $300
AEM Price $285
Airaid Price $305 They do sell just the tube and filter Airaid Jr. Kit or Just dam and filter for less money and claim same numbers, half the money
AirAid Jr. Kit Price $156 (*Just filter and Tube, uses factory Air Box*)
True Flow Price $245 (*Just filter and tube, uses factory Air Box*)