Interesting comments and thanks for the feedback. I'm still undecided....
I'm concerned about the strength of a US box fin. When it is fully loaded, is it not as strong as a bolt thru type?
I have also considered changing the fin box on the board to a tuttle - more expensive option, but I need to be realistic about when and how I will use this board. I suspect it will be (as yet unridden) 15 - 22 knots with KA Kult 5.8, 22 - 35+ knots with KA Kult 4.7.
The idea of running a Koncept on this board is driven by my enjoyment of speed and ramps


and I figure I need a fast fin to suit the sail type. I guess a Koncept won't work with a wave / swept design? So I need a slalom fin around 28cm that will still let the board turn, but have enough power to balance against the Koncept.
Also in my mind are Spotty's 40+ knot efforts on his larger Kode, so the board is obviously capable of some nice top end speed.
The successful fin will give me good bottom end, balance against the board and sail type, and good upwind (yes, all on a bump board). For outright speed, and light air sailing I have other gear covered, so it's not as though I want this board/fin combo to cover everything.
Kimba showed me this one:
http://www.sunshinecoastsailboards.com.au/Components/Board-Components/Fins/Select/Select-Proam-Freeride-S.htmlAnyone know much about it?