Yup that's exactly where I'm at, K&N cold air and the Diablo. Just made my appointment to have stainless 3.5" exhaust run from the down-pipe back (going to eliminate the 2nd tiny cat - esp since the 02 sensors are just above the bottom connecting flanges on the downpipes.) which the ford tech at my local shop projects should add another 40 hp (putting me damn near 500).
Need to find a good boost gauge and controller - and really hoping to find a way to mod the top left center console air vent to fit the gauge in there, similar to the one's for Audi's;
http://images.channeladvisor.com/Sell/SSProfiles/12001428/Images/2/Audi_A4_2.jpg