I sure ain't gonna argue with old smokey about momentum - other than to say my little plane will have less momentum that his big one and is therefore more suseptable to wind changes. Of course, conversly, my piston engine can add power quicker than his jet because it doesnt suffer from the spool up lag that jets do (am I right in thinking it takes typically around 5 seconds for a jet engine engine to spool up to maximum power?). Of course, the power he can generate makes my piston engine look like a toy, although I need less of it!
As for the jet stream question - (and again standing by to be corrected), part of the job of a commercial pilot is to try to run the aircraft as economically as possible so if he can jump into a jet stream and get the same speed for less engine power (therefore lower fuel burn), he's gonna do it. The thing about windsheer is that while it always has an effect, it only becomes a major problem at times like landing when you are closer to minimum speeds and the ground.