That is a very neat design. Just like the way heatercores work, must have taken quite sometime to mill. Maybe if you made it a little thicker, might work even better. With a 156 watt pelt, thats about the temps I would expect.
I have compared my rig to a few others at tekheads forums who are using either 220w pelts or dual 172s, and my temps are quite a bit better, my bet is on my waterblock. You can see the comparision graphs here.
http://www.tekforums.co.uk/posts.php?threadId=7189
My rig set up is here.
http://www.overclockers.com/tips800/
I have since switched to AS3 and temps are about 1-2C better, not a lot but enough to note a diff.