Ken, you say the P30 isn't too bad with long exposures, I haven't found much information on this? I've looked around on Google but find mostly P30+ info. I don't imagine any of my exposures would stretch out into multiple minutes, 30 seconds would usually be the extent of it.
Thirty seconds? That's it? P30 can do it without breaking a sweat---as can all the P series backs.
IMHO, the P30+ wasn't much of an upgrade of the P30. I'd place bets that the latest C1Pro 7 will have the greatest impact on image quality because of the new algorithms. Of course, that goes for the P25+ too.