4.5m is pretty small for even the iS87, and a solid 25-35kts can be a real lot of wind for "comfortable" (as opposed to "serious") use of a slalom board. Which confirms for sure that the iS101 (sweet though it is) is not the ideal board for this spec. You would find a serious overlap everything but the lightest wind range of the iS115 and the 101 won't give you such an "easy" job of it in the 25-35kts semi/sub nuke end of your target range either. For 80kg sport rider, the 101 range is around 12/13kts to 25++, but the sweetspot 15-22+ or around 6.5 to 7.5m in race sail sizing (+/- the usual variables for rider/condition variables etc). On flat water you can extend the wind range in terms of easy control, but in turn if you are going for top end (broad angle) drag race or GPS speed in these higher wind/flatter water conditions, then something like the iS87 will (at that point) have an easier top end advantage over the iS101.
The 101 is a really fast, versatile mid wind slalom board, but in this case we really look like discussing a hi wind slalom, which is not the ideal (or easy ride) for the 101.
Hope this clarifies it all, but don't hesiate to ask away for more info either
Cheers ~ Ian
Cheers ~ Ian