Just to be a contrarian, this advice goes somewhat counter to my experience. Esp. playboating a lot of water can come in through the tunnel so I'd prefer a fit a little too tight than too loose. I also find (with exception of Snapdragon skirts) that the neoprene around the tunnel tends to loosen up a bit over time. I've not found that the velcro on the drytop/drysuit adequately compensates for a loose tunnel. I'd be more inclined to size down. Your decision may also depend on how many layers you tend to put on. I imagine in San Marcos you tend layer pretty lightly.
The cockpit fit is a lot more than about size, as you've already noted. It also has to do with the rand or bungee used and overall shape. I think you've got the right idea asking what skirt people have found to be dry on a Rockstar. I don't know the answer, but maybe searching the web would help. The good news is that the Jackson playboats seem to have a nice deep lip where you can get a good seal. Sizing down on the cockpit doesn't necessarily make for a better seal, though I would tend to agree that I would favor getting a better seal over ease of popping it on/off the boat.