The double thickness PA120.3 is performing better across all fans, with it's greatest advantage with Ultra High Speed fans.
So, as expected the double thickness PA120.3 is superior, but the difference is smaller than I was expecting. From my results I only measured somewhere between 6-14%, and I had expected it to be more in the 20-30% range, based on forum discussion and a few reviews.
I have to question my results as I just don't like surprises without checking it and still learning about this type of testing. To do that I went back to Thermochill's site and looked a little more closely at their c/w curves at the bottom of their technical page. This testing was done by Bill Adams who I've followed in his work and respect and trust his results moreso than my own, he really is the king of testing and knowledge. What I found was the very bottom c/w graph just so happens to compare and contrast an MCR220 and a PA120.2 on the same curve. Granted that's a double to double comparison, but I would still expect their relative performance percentages to be similar or close. Using the Nexxus fans (which is essentially yate loons at 1000RPM) I could extract roughly the following values:
PA120.2, c/w = .0475, 10C delta = 211 watts
MCR220, c/w = .05, 10C delta = 200 watts
To calculate the 10C delta, you just need to follow units by taking the inverse and multiplying by the delta. For example if c/w =.05, then w/c = 1/.05 = 20 watts/celsius degree. For 10 degrees, that's 20 w/c * 10c = 200w.
While it's hard getting anything very accurate from reading a chart, assuming it's close enough for a rough check...that still calculates out to about a 6% difference, and I measured about an 8% difference with 1000RPM fans on the triple radiators. So overall, I'm content with my results meeting a reasonable check.
So.... with lower speed 1000 RPM fans, the PA120.3 will dissipate somewhere around 6-8% more heat than the MCR320, and that percentage raises up to about 14% as the fan performance increases and the extra thickness is better utilized. The MCR320 is really a great radiator for it's compact size and performs much better than I expected. I attribute this to two things, one is the larger frontal surface area than many standard triple radiators, this extra frontal area provides a great benefit. In addition the MCR320 (14.6FPI) has about 40% more fin density than the PA120.3(10.2FPI) per my counting across the length of the radiator. I didn't notice this upon initial inspection, but this prompted me to count fins across the entire length of the radiator. So while the PA has the additional surface area with double the thickness, the MCR makes up for some of that with more surface area in the first 21mm thickness.
And to take this information and apply some assumed heat loads, it would look something like this: