Thanks for the very informative response Vando.
I must say though that I have mastered* the S-type 93 and 6.7VMG with the 30cm Select Viper. It will spin out if the wind drops releasing mast foot pressure but I seem to be able to correct this pretty quickly. If I put the 34cm Drake in it feels way too big now even with the 6.7, although it does take some work to get planing occassionally in the lighter stuff.
Can using a smaller fin than recomended hinder your sailing progression?
Also, I've just received my new 09 iS 122 (113litres

)with the two stock Drake slalom fins. Haven't had a chance to give it a go although did you find a massive difference in the supplied ones to the customs you use?
*'mastered' used figuratively