JimS,
Thickness contributes directly to surface area of fins. Obviously it only contributes to tube surface area if you use the additional thickness to fit in larger tubes or multiple rows.
Thickness has a diminishing return regardless of radiator size simply for the same reason that the fins/pins on air cooled heat sinks show diminishing returns. The greatest contrast between fin and air temperature occurs where the fin meets the tubing. Because the conduction coefficient of metals is so much higher than the convection coefficient to air, you don't need much metal (thickness) to conduct the heat from the tubes. What you do need is lots of surface area and the closer that area is to the tubing the better.
Unfortunately, it's the weekend and my heat transfer book is at work. There is an equation for fin efficiency based upon conduction and convection variables. I can post that next week if anyone cares.
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