Dunno....Maybe
It's been quite some time since I flew 'The Guppy'.....Have no idea
about Sitka, the runway length, the surrounding terrain, etc.
Usually, we used improved climb on hot days...or situations where we
had a lot of runway, but were climb limited.
In this case, maybe due to terrain, they use high speeds....assuming
they have the runway to do so....to get better OEI climb performance
in mountainous terrain.
If that's the case, 150 KIAS is not an unusual V1....