RE: iS145 vs. iS133 in 8-14kts 7.5-9.5 sails?
No No No - don't!
There is no doubt that the iSonic (for instance in comparison with a similar size Fanatic) planes a little later. The joy of the board comes at the top end when it remains controllable for much longer.
To get you planing a little earlier don't move the mastfoot back! If anything move it forward to keep the board flatter for longer. The board has enormous sail carrying capacity, which means that the 9.5 is just about middle of its range. I stick my mastfoot slightly forward of the mid-point with my 9.4 lightning, and about in the middle with the 8.2 Redline ( I wouldn't stick a smaller sail on it).
Once it is planing its stays there for some time and its use of apparent wind is beautiful. The trick is to get there, and this will require developing your pumping technique. When you spot a little gust, point the board slightly further downwind and work those arms and legs.
As regards to the footstraps, you can play around with their location on the rails a bit with the existing holes and I found that there are (for me) positions that work better than others. My backstrap is back as far as possible and my front strap forward as far as possible, but then I am quite tall. As with any slalom board it requires a bit of commitment to get to the backstrap. The key here is mast foot pressure. When you are about to put your backfoot in, pull down on your mast hand so that you load the mastfoot and front foot. This will allow you to free up your backfoot without going up wind.
Don't mess with extra holes, its not necessary! I am a size 12UK (46/7) and I can sail this board comfortablly. Just make the straps nice and big.
Hope this helps