Hi Dennis,
It appears to be the case that Groove Quantize ignores that velocity setting (which is more of a multiplier) and works strictly from the groove velocities. It doesn't appear to be a bug, but perhaps more a design choice.
It may make for a good feature request however if it doesn't already exist, to have the velocity slider also work with Groove Quantize.
http://answers.presonus.com/studio-one-feature-requests