I just handled on of the new 20ga Ultra Slug Hunters w/thumb hole stock at WGS. I also have a 20ga Ultra Slug Hunter and with the weight removed from the stock I don't find it that much of a chore to carry. I purchased a 20ga Tracker II barrel from the classifieds last year that I planned to set up for hunting the thick stuff (corn), but have yet to shoot it. I think I would be better served with one of the Tracker I smooth bores, as with these narrow corn rows today most of my shooting would be under 25 yards.
I purchased two of the thumb hole stocks and the forest camo one is just like the one on the Ultra Slug Hunter, Pachmayr Decelerator pad and all. The only difference is mine is a blem, has a void in the laminate. The t-hole comb is low enough for sights, you are right about that.
As far as what 20ga barrel would suit you best, it would have to come down to what type of huntng you will be doing with it. If you will be hunting mostly from a stand, you will want the Ultra. If you are a still hunter, you may like the Tracker II better.