OP you're getting bad info here. There is no aileron droop with the Horton, so this comment on the 6-300 isn't applicable either, or it's been mis-remembered, or it's something else.
The major portion of these STOL kits is the leading edge cuff and in later years, Cessna updated their wing profile to add their own cuff that I think they called the "camber lift" starting in the early 70s. The camber lift had almost the exact same profile as the Horton STOL leading edge cuff, so it's really not a dramatic difference. See the graphic here:
https://www.steneaviation.com/collections/sportsman-stol/products/sa-sportsman
I wouldn't expect much change in handling at all, top end or bottom end, and definitely wouldn't rule out an otherwise good plane, if you've found one, because it has a Horton kit on it.
FWIW, I have the Sportsman STOL kit on my cessna and it is amazing. It's by far the most dramatic cuff and it doesn't affect my top end speed compared to similar planes without it.