RE: bigger fin for EVO80
Personally, I use the stock 23 fin on my '06 Evo 83 with 5.8. It seems to me there is no problem, at least at 85 kg..
I doubt that a bigger fin would be of any help in planing sooner; while surely it would help in tracking and pointing.
I think that for your weight the 80 turns out to be an higher wind board compared to my 83 for my weight; so you should be able to use it faster and even better with the stock fin.