You will not have a hard time hitting 80. I have bigger tires and more weight and can easily go up mountain passes fully loaded with RTT, 4 adults, stuffed inside to the gills, spare parts, full fridge... and maintain 70 on a 5-6% grade in 4th (yes I am over 3k-3.5k and as much as 4k rpm which only affect mpg not the engine). The top end that you lose is probably the part that you don't want to get to in a lifted, modded fat pig. Easily can cruise on the interstate at 80-85 but your revs will sound fairly loud and mpg will suck more due to the increased rpms.
The biggest difference is that I don't get much better mpg on the road (its better but not a huge difference). I hover fairly consistently around 14 (manual calc because the gauge is off by about 7%). One strange thing is that with 34s it is better than it was on 33s. on 33s I would get slightly less MPG but more throttle response. FWIW, I drive for the most part averaging about 75 because I just feel safer going slightly slower than I would otherwise given my weight, center of gravity and lack of sway-bars.
PS. when I'm on the freeway, hands free cell phone is a joke. My rig is so loud people can't hear me well. lol
) .... I did not go for the manifold as I have no plans to run lockers.
