That machine is 2 pounds heavier, half again as thick, and has a shorter battery life; in other words, it's more like the cheaper model 15" Pro, which is also easily upgradeable, and also an inch thick and lighter at 5.5 lb.
Every MBP since the unibody design is 0.95" thick(of course except for the new retina one) and the T420S is 21.2mm-26.0mm thick, how is it slimmer?