I suspect that the 20 lb. limit is over cautious, but I can't say that with certainty because I don't know how BRP came up with that figure. But at the same time, I have to wonder why that weight limit for the trunk is so low. That rear storage module is a fairly sturdy piece, and it is bolted to the frame with some hefty bolts.
My greater concern about loading up the "Show Chrome" trunk rack is that the way it mounts to the trunk lid might cause stress at the attachment points. Some folks have reported cracks there. That is partially why I got this rack from Value Accessories instead. (The other reason was that it doesn't hide the high brake light.)
When going up for SpyderQuest last fall, I had that trunk rack loaded with a medium size trunk bag, and also a fully loaded backpack on the passenger seat. I had a little room to spare, but not much. i would be fairly comfortable on any extended trip riding solo in this fashion. But frankly, I don't know how folks who travel double on the RT do it without a trailer.