I sailed the K87 and have the 08 K79 now. Both are terrific boards with a similar feel ad the K79 is clearly a size smaller. If you like the K89 you will love the K79.
There is one option however: the Pure Acid 80. It has a very similar rocker to the Kombats and are just as fast. You'll miss out the double back strap option obviously. Except for that, the biggest difference is that the wider tail of the K79 will support it better at slower speeds. The narrower tail of the PA80 will make it more controllable at higher speeds. On flat water, I would say the range of the K79 will be slightly bigger. The wider tails is not much of a problem even in windy conditions here. But on a wave, the PA80 will have the wider speed range. When it is windy or if you go fast on a wave when riding, you will have an easier time handling the fast rocker with that narrower tail.
But again, the K79 is a great option and if you don't ride wave at higher speeds, probably the best compromise, particularly if you're closer to 90 than to 85.