Not really lower. Maybe a few degrees. The limiting factor in the "all in one" cooling systems is the pump speed, and lack of a reservoir over a full out water cooling loop. Most of the all-in-one cooling systems perform within the same few degrees of each other, especially if things like the fans are equal. They do a decent job, and keep the heatsink out of the way and gives that cool water cooling look to it, but in reality, they don't cool much better, if any, than a good heatsink/fan combo. They just look cooler, and are all the rage right now, but they all suffer from having a small pump in the waterblock.
FYI, I have a Seidon 240 all-in-one watercooler, so I'm not trying to really trash them, but wish I researched more first and didn't fall victim to the OMG, it's water cooling, I'm "leet" marketing and spent the extra $100 on my video card instead.