RE: wave boards?
Even at 80 I think the 74 can take a 5.5, especielly if you're used to sailing a 68. The 74 feels quite a bit floatier and more stable in low wind, though I guess the actual volume difference is not more than 5-6 liters. I actually think these liters will make more of a difference for you at 80 than they do for me at 69. The PA74 is quite a bit wider (2cms) and that helps too for schogging. I would stiil say it is easily drivey enough though. We had a narrower proto for the 74, but the wide version that went into production was just much better balanced. I think the slightly curvier outline and drivey bottom shape complement each other perfectly and gives the board a very wide sweets spot wrt which waves it handles.
Normally I would say that for 15 knots and 80 kilo, you should go for the 80, but again, since you're coming from the 68 and have the low volume thing wired and since the shape of these boards kind of carry weight very well when it comes to the actual sailing, the 74 will probably be a big hit with you. Kevin Pritchard (around 84) was ripping big time on it during testing also in light wind (I think he was using a 5.7 on it even).