-A penny (brass) and TO-220 (tinned copper if I'm not mistaken) have different coef of thermal conductance, deltaT per W. Well might be that tinned copper is a better thermal conductor than (an oxydized) brass, have to look in the book
-The surface could be twice as big, but also could be much more - all you have to do is to scratch the penny with a Dremel or a rough file. Surface could easily go up tenfold, so the thermal conductance. I love to scrub the paint and scratch the bottoms of the heatsinks in my computer
-From my expirience, if you don't use smth like Arctic Silver (which I do), the contact b/w the penny/heatsink and the chip will be a major bottleneck, no matter how large of a heatsink you're going to get
-I'll be curious to see degradation curves for some of those chips, but from my (modest) expiriece electrolytic capacitors are suffering from heat the most, while output transistors can run very hot (above 60C) for many years with no audiable degradation of the sound
Just my 2c
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