Another factor to consider, if you increase the fluid volume of the radiator (by adding extra size/square area, rows of fins, etc), then that extra volume should help to dissipate at least a little more heat.... my thoughts.

Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!