I can't tell you the HP gains but I can tell you seat-of-the-pants feel of a K&N and exhaust.

When I first did the lift and 30" tires every thing was stock under the hood. I have some good hills to climb on my way to work so this is what I noticed. Stock I lost about 10 to 12 MPH on one of the hills in 5th gear. I did the open K&N first and on the same hill I lost about 8 to 10 MPH. With the 2 1/4" exhaust and a MagnaFlow muffler I lost 6 to 8 MPH. I put a big hole in the MagnaFlow in Moab and went to a Summit Racing Turbo muffler and went back to the 8 to 10 MPH loss. With the 31" tires I shift down to 4th at the bottom of the hill and gain about 2 MPH.

In my opinion the filter or exhaust by them self gives a marginal gain, with them both together you get a better gain (especialy in a V8). Also a good muffler is better than a cheap muffler, I have never gone wrong with a MagnaFlow. Flow Master are good mufflers also but I hate the way they sound and a Ford V8, they sound good on a Chevy though.

Just my thoughts, Dennis