Horton STOL 172 "V" speeds

I've just bought a '62 Cessna 172 with the Horton STOL kit and madras
droop tips and have a question:
My dad took it up for the first time (it's in WI, I'm in NC) and was
curious about if these modifications change the "V" speeds in any
significant way...? Apparantly there is no change in the POH after
these modifications, but obviously there must be some difference.
Specifically how Vx and Vy would be different, best glide, and what to
approach at. He and the instructor were using 75-80 as the approach
speed (which seemed high to me). Does the sink rate increase noticably
with these modifications?

Thanks for any insight!
Ryan Wubben
Chapel Hill, NC