Hi Will, can't answer your question but do you have a brother called Max?

Will, I should be able to answer most of your questions re Koncept. Before doing so I would like to know what type of racing you want to do, I assume you mean slalom??

As an overview the 2013 Koncept has 2 personas, the smaller sails up to and including 6.2 are more geared for downwind speed sailing, the 6.6 up geared for more all round high performance sailing. The difference is in the shaping the smaller sails have more shape down low and the shape is more forward placed.
It is a fast sail and holds it own against full blown Race sails, the pros being that it is lighter and with the narrower sleeve easier to handle with a softer feel, the cons being it starts to lack the stability a full blown Race sail has when sailing in very overpowered conditions. It also has a fairly full entry so is not quite as efficient as for eg. the KaRace when sailing tight or upwind.
It will work on the 55% but will feel noticeably more responsive and lighter on a 75% or 100% mast. I honestly would not really recommend the 55% for racing but it would get you started until you got a more high performance mast.

I hope I have helped and PM me if you want more specific info.