RE: wave boards?
Sorry for my delay, I was travelling around. I'm afraid I cannot elaborate much as I just did a quick spin on the K79 of a pal with my NP NR 6.2 sail. Wind was 15-20 knots. I was on my K86, 6.2 sail and 23 cm wave onshore fin (North Shore 9.0", much more surface than the Drake natural wave 24 cm fin; *). With my 65 kg, the NR 6.2 covers nicely 15-20 knots, so really I don't know for your heavier weight.
As I said above, the K79 stock 24 cm crossover fin is fine with footstrap's wave setup but too small with footstrap's freeride setup (lots of spinout) with a 6.2 sail. Then, the K86 stock freeride 30 cm fin felt fine on K86 but is a bit too big on K79, especially during manoeuvers (tight turn, slashed turn, tight jibe). K79 felt one size smaller than K86 for sure, especially with the thinner rails and narrower tail. My gut feeling is that you can even try a 6.5 sail on K79 (with 28-30 cm fin), but not much, owing to it's limited tail width. I regularly use K86 with either a NP RS1 6.6 (15-20+ knots) or NP Jet 6.9 sail (12-18 knots; both with stock 30 cm fin which is king) without any problems.
Agree with Ola, you can try a 6.5 sail on PA80 since you're pretty much feather weighted, but to increase the bottom range as well as decent pointing upwind, a classic 24-25 cm wave onshore or Drake Natural 26 cm wave fin (**) might do the trick to cope with the extra power from the sail.
(*) K86 report : http://2006.star-board.com/forum/askteam/read.asp?ID=5804