I'm 86kg and I use a Kombat86 + Acid74 (so not the Kode but very similar with regard to wind/sail range). Both 80 and 74 will carry 3.7-5.0, I think you have to decide based on these things:
1. K74 will be better when well powered with 4.2 and 3.7
2. K80 will make a more comfortable overlap with your K94, and better in holey 5.0 conditions
I'd say go for 74, it's not that small for 78kg (in neoprene, so you're 75 or less).