For me, a 39T middle ring seems to hit a sweet spot. On bikes with a 42T middle, I have to shift to the granny earlier on some tough climbs, while with a 39T I can hang on a little longer. True, not a huge difference, but on short steep inclines sometimes small differences are big.

I've run a 30T rear cog on a Campy medium cage RD as well without incident. Why does Campy call it a medium cage? To differentiate it from a MTB specific RD? For kicks, I measured the center-to-center distance between the pulley wheels on a Campy medium cage and a Shimano 105 GS, and they are both about 8cm.