RE: Hypersonic 105
I'm glad for your nice deal!
I think Ian Fox has a clear explanation as for why the Hypersonic line was stopped; well, perhaps we could say that it somewhat evolved into the moddern wide hull iSonic line. Personally, I think that it did not have much of the classic "slalom feel" but rather it is/was a great sporty high performance board, only a bit too much "sporty" and requiring. Many would not like it, a few would just love it, in the end it isn't for everybody.
Don't worry, learn how to tame it and you will find it perfectly suits the needs you stated in your first post, probably better than any other board would.