MPG change after cam swap

I'm putting in the TFS1 cam and am wondering how much my highway mileage is going to decrease. Any of your personal experiences are appreciated! My mods currently are: Stage1 ported TW heads, Typhoon manifold, and stock cam w/1.6 RRs. I'll be adding the TFS1 cam, 30lb injectors and possibly sending off the lower manifold to Extrude Hone or buying a Victor 5.0 intake to replace it. I'm thinking about buying a Victor because I'm also getting a built 306 soon that'll be a real spinner pretty soon.

Thanks guys, in response to what ya'll said, I would only be using the TFS 1 until I get a new shortblock, after that I'll upgrade the cam to a solid roller and put in some Comp Beehive springs w/Ti retainers and locks for the valves. I'm looking to upgrade the intake right now because it feels like the one I got now is running out of breath; it looked that way to me when I put the heads on that the Typhoon doesnt flow nearly as much as the intake ports on the heads but was strapped for time then because I had to go on a 2000 mile road trip soon after.