You're my weight and I sailed K87 last year and K79 this year (ie 2008...). I also have EVO 70 an S101 btw. I would say that between those, the K79 makes more sense as it connects better to the E70. The K79 takes a 5.7 quite well. But if flat water blasting and B&J sailing is what you mostly will use the kombat for, then you could have a look at the 89. It is still a nice board in waves albeit not quite as throw around as the 79 when it gets a bit better/bigger/faster. Powered up 5.2 is no problems from a control perspective. You also get a more stable platform for tricks and stuff and maybe slightly more effectiveness with a 5.7.
So in the end it depends a bit on what you want to focus on with this board. If you want to expand your possibilities in wave, the 79 makes more sense and if you want something maneuverable for mostly B&J the 89 maybe makes more sense. If want a bit of both....well both boards will do the job well so its just a matter of picking what you believe the most in. One thing that talks for the 79 is that if you sometimes sail high wind and no waves, the 79 is very, very fun in such conditions.
I hope this helped. Otherwise, just ask again.