I'm 6'-2", 180 lbs, 35" inseam, and have a one-piece Roadcrafter size 44L, in standard cut, without the lower back ellipse. I ordered the the Competition Back Pad in size large, and received it a month ago. I chose the Competition version for the extra protection it would offer.
Though I was concerned it might hinder my forward lean excessively, it increased that constraint only moderately. As with the TF3 knee, elbow, and shoulder armor, the Competition back pad is just a slab of blue foam (though a very particular, energy-absorbing blue foam) with a black, plastic outer layer, but my impression is that it's well suited for the everyday commuting I do.
In terms of thermal comfort, as I've only been using the pad for a month of spring weather riding (temperatures between 50 and 80 degrees Fahrenheit, with moderate humidity), I can't speak for its performance in full summer heat. So far, however, it seems to have had little effect on the suit's ventilating effectively.
One minor fault might be that for my torso length, at least, it seems that the pad could be longer by an inch or two, better to cover the tailbone. (Aerostich might be able to do this most simply by extending the pad's tail by that amount.) In my case, in an attempt to get better coverage of the tailbone, I installed the pad an inch lower than it's designed to go. (I purposely misaligned the pad's hook-and-loop rectangles an inch downward from those sewn in the suit.) I believe my torso length is near average for someone of my height, though I'm not certain of this.
Other than this minor question of sizing, then, I'm happy with the purchase, and I think the Competition pad will serve me well.
(Posted on 5/15/12)
Living in Scotland I expected delivery to take a week or two so you can imagine my surprise when I received this on Friday morning after ordering it on the Monday at gone 9.00pm in the same week! Back pad fitted easily to my all in one suit and also to my leather and also my cordura jackets. Fits like a glove with no interference. Very Impressed. (Posted on 11/7/11)