Yes, lots of overlap
If you love the 115, you'll be ecstatic over a good 101 session.
Yo're definitely correct, the 101 is close to the 115, about as close as the 115 to the 122...(seriously, the performance size gap is about the same).
The iS87 definitely delivers better control and "power on" handling than older 85 Lt designs, making performance more consistant and less peaky than the older 85Lt rockets (which tend to be a bit cammy, fully on or not so on..and more of a handful at full noise thru hi wind chop) iS87 is definitely OK on upwind for a board this size/style, but noting that obviously the performance envelope is focused towards downwind, broad reaching speed rather than upwind performance.
Cheers ~ Ian