RE: Isonic 111 speed
I'm pretty interested in that as I'm just waiting the 111...
In my opinion you have to consider that 30 kns speed wall it's not so easy to overtaking and one important factor to better understand your feeling is how much wind did powered you in that case.
When it comes to fin lenght related to board/sail combo a very important thing to think is to know if you wanna blast or what else.
Thinking at purchased fins,please,consider that they are supplied for keeping the wider wind/sail range possible so,maybe,it didn't match the condition you've found.Plus,I think,you can't expect too much in top range performance by these fins.
If you wanna reach a high speed in 7,0,according on what Remi suggested,you should try a Debo sl2 36 and use a cambered sail.