Now there's a mistake ! Please accept my apologies, Jeroen (and others), that was definitely a iS vs ST mix up on this one.
So, to talk about the iS
115 with 8.4/7.2m..
(Actually, that is even an easier reply, too !!)
Footstraps : For 8.4m lightwind speed, and as long as you don't experience any major control issues over chopconsider to use the very back positions on the Front and Rear straps (subject to height/strap pitch). For really powered or choppy 8.4 and most 7.2 conditions, you will likely find the mid strap position a more verstaile setting as it allows a more "calming" stance over the board thru the chop, which is usually no less speedy (often more..) but in most cases easier for the rider to stay on the power - and in most cases gives a more fluid jibe with a more engaging action on the rail in the entry/mid jibe.
Mast track. For 8.4m speed, you can use 50-60% forward (this one depends also on the rider height/boom height, so don't be worried if your ideal seems a little one way or the other, but in general the basic mid position is still close to the best for 8.4, and definitely for 7.2m, the mid (50%) or even just behind if you really want to tweak for more top end speed (if the water/chop allows for that). Again, in both cases if you are REALLY struggling for control, it's ok to force the situation by moving the foot 1-2cm forward, but in general (if everything else is close to good) it should not be necessary.
Fins : Typically using more than 42cm doesn't add significant early planing, and becomes quickly a law of diminishing returns when considered against top end speed and control (which usually suffer noticeably above 42cm).
It can definitely be worth considering a smaller fin for powered 8.4m conditions, as well as a larger fin for "marginal" 7.2 conditions - in this situation the size choice can also be influenced by "practical" factors. One additional fin at 38cm would cover the "overpowered" 8.4 conditions, as well as doubling for "marginal" 7.2m. For pure downwind or speed in powered/overpowered 7.2, you can consider a 32cm, but keeping in mind the mid size iSonics generally perform best with a moderate (not excessive nor too minimalist) amount of fin for the conditions. Of course, it's possible to go crazy and have every fin at 2cm gap, which is nice - but in most cases excessive. The stock sizes are very capable and in typical sailing range well spaced to cover most of the effective range of the board/s quite well.
Make and model of fins is influenced by rider style/preference, as well budget and availability, but in general for iS in this range look for medium stiff fins (in this size range), with fast foil profile and low rake angles.
Hope this clarifies it, but don't hesitate to contact us for further info or questions.
Cheers ~ Ian