Coldplate question

OK - I've looked at the "coldplate thicknesses" article on the overclockers guides, and it's not displaying correctly or something, as I can't get the pictures and graphs up.
My question is with a 226watt tec, and watercooling, I was wondering whether to stick 1/8th inch - 3.26mm copper plate on it, for the coldlpate on my XP2700, or whether it's worth machining my own coldplate out of 12mm copper - taking it down to about 8mm in the process. This is due to the copper stock I can get hold of, as it's 12mm thick x 50mm wide, but has rounded corners which are just enough to cause the pelt to thermally short on the outside, therefore to get the full 50mm, you have to take a millimetre or so off each side. That's not a problem in itself really, I was just pondering. I probably will end up machining.

I'll post the preliminary idea (quick 3d pic) I've got for this if anyone's interested or wants to comment?

As far as I remember the thicker the better, but you get really diminishing returns somewhere after 7-8 mm. I think 1/8 inch ain't too good with a small-die cpu like athlon.