The rise will be water temps finding equilibrium (what are the water temps btw?) You might knock a few degrees off with a better rad setup or not depends (btw 7200 rpm fans sound excessive, mine is single and can push 235 cfm at 3500 rpm
)
As for temps, I don't think -10C is realistic (wtf you get that number brad?). I tapped your numbers into a handy peltier calc and came up with around -5 to 13C as realistic temps (note this takes into account peltier voltage and such as well as cooling system efficiency). -5C is with a damn good setup on the other side. This is assuming those PSUs are actually putting out 20V.
As for rat shack vs artic silver, AS2 is know to have major problems with foiling at low temps, you may do better with generic white goop.