I was on the same thought process several years ago. Don't go down that path, chasing KV size. In my Bismarck I have a 1000KV motor on 3S batteries and a 10BL60. Could not be happier now with that set up and size boat.
I'm looking at a pair of Tenshock RC906 in 1250kv for my next class 6 build where I'm planning on going 12v. Most of us have gear trees with pinion gear sizes (32p) from 11 to 22 or so. Assuming a 28t spur gear (on the drive shaft), I usually start with a 12t on the motor and run it. If you're up near 20t in pinion on 32p gears, you're probably needing faster or bigger motors. There's no real science here that will tell you to run X gear with Y Motor and Z prop. There's a point where you just put the ship on the water and start experimenting. Ship is too fast, smaller pinion / less %esc. Too Slow - bigger pinion / more %esc. You'll get different settings for different ponds (or even the same pond at different times of the year - water is wierd).
