I did Flexalite fans on a 6.0, No noticeable power increase. the biggest difference was in the AC, at Ilde it'll freeze you out, where before at idle it hardly blew cold at all.
If you go with Flex A Lites and use their "VSC" (Variable speed controller) Be sure the controller you get has wires coming out of it "NOT Spade connectors" The Spade connectors come loose from the board, The VSC cost $90.00, I have had 3, Now I just use 2 simple temp probe & Relays I bought from Summit for $20 each wired individually to each fan, if 1 fails at least the other will work, if a VSC Fails you screwed cause it controlls both fans (unless you on the highway).