As an owner of is101 07 model I can tell you that it works very well with 6.2 race sails.
But, I find 7.6 to big for it (I am 83kg). I am much faster with 6.7 sail as soon as I am decently powered. 7.8 will be much better on your is125.
I think that 125-101 is great spacing. 101 is very controllable in extreme conditions and I think you will use your old wave board less and less.