I have the Is101 and Is87 and the combination is brilliant. Width is much more important than volume. The Is87 is 56.5 wide the Is101 is 64cm wide, however the Is87 is much narrower in the tail.
I use the Is87 with a 5.7, 6.3 and 7.0 North WARP, the Is101 I use with the 6.3, 7.0 and 7.8 WARP. Although there seems to be an overlap, in reality the boards deal with very different conditions. If the wind is gusty, the Is101 is better for racing as it carries more speed through the gybes, however, if it is choppy and full on then the Is87 is a much nicer board to gybe
In one slalom event this year, we had winds up to 45kts, I was using a 4.5 North ice on the Is87 and it was fantastic, conditions were short chop about 50cm high (ie horrible), I used a 29cm Select Caspar Speed fin, which was surprisingly good in those conditions. I am 174cm tall and 85 KG. I bought the board from a starbord rider, he told me that he wished that he had the Is87 for that event rather than his new Is90 as the Is87 is a better board in really choppy windy conditions