The operator I flew Vans on meat bombing ops used 105 KIAS for the climb to jump alt
Must've been using 675hp on the way up from what I have gathered. Or they missed something other skydive 208 operators didn't.
Start with the question 'why does Vy decrease with altitude' and the answer lies within. (and yes, caveats for turbines with low flat ratings/high thermodynamic rating or turbocharged pistons)