isonic 87 2007 VS isonic 86 2010 and sail problem

[IOANNHS]

I own an isonic 87,model 2007.I use it with a 6.7 code red and 32 cm techtonics talon fin.Thinking of changing the sail and might change the board also.The problem is that the new severn race sails have sizes 6.2 which is small for me and 7.0 which is probably a bit big for the new board since the sail range is max 7.0.From my experience,i never use the bigger sail of the sail range of the board.What do you recommend?and with what fin?Even if 7.0 is working great with the new board,i should probably use a 36-38cm fin.So the other question is this.....why should i be faster on a 7.0 sail on isonic 2010 model,with a 36-38 fin and not with a isonic 87lt,model 2007,with a 6.7 code red and a 32cm(which i use now)..