Ben, I did a lot of research before buying the Litespeed and got to the conclusion that it's supposed to be slim fitting, this way you can avoid flapping in the wind and allow a better vapor transfer, if you need layers you put them on top when you rest. I bought my regular size L (6'2 height,42 chest) and it's comfortable with a baselayer and a microfleece , went as far as wearing it on top of a 300 fleece , it zipped up but just barely. The neck is tight, don't know if it's intended to be so.
Patrick, thanks for the input but I can't buy Rab for moral reasons, I guess I'll be sticking to my 40$ Land's End synthetic jacket (2,2 lbs!) till I have enough money for a Westcomb or something...