upgrading the fins
really depends on what you're trying to get out of the board. If you are mostly freeriding, than the stock fins
will work fin and might make the raceboard learning curve a bit easier.
If you want to maximize the performance of the 183, you'll want to get some good G10 fins
in the ballpark of 34cm long front, 28-29cm long rears (running the standard quad configuration). The chord length at the base should be around 9-11cm...10cm is most common. The cant angle of the front should be about 6-7 degrees. The rears should be straight. Rake angle is about 10 degrees.
if you have no idea what i am talking about, check here http://kiteracing.com/Fin_Terms.html
These bigger fins
will allow the board to heel to leeward more and really let you drive upwind. Downside is that they are bit harder to ride and take getting used to, but once you do, you'll never go back. We might have some prototypes available in that size but i'd need to check. If i run across something I will post it here.