Chad,
I have had the jetstream lite, my son also has one. I now have the timberline. The jetstream is cut much tighter through the body than are the mountain and timberlines. It is made to fit tight. In my case, that was good because I could go up a size and it still was nicely fitted, but was long enough for me (i am 6'3"). In a timberline, I needed a large for the girth, like I do for most Sitka stuff, but an xl for length, so my timberline is a little baggier than I would like. XL in jetstream fit great. All that is to say, if you want to wear anything under it, you want to go up a size larger than you would normally. HTH...