There is nothing amiss with the behaviour - the minimum mass (so fastest speed) for C7 DD/Drag drives is 945tThere must be kind of bug with its Speed(Top/Boost) and Pitch/Roll/Yaw, changing weight have absolutelly no effect. Weight for equipped I.Corsair distro is 80t
Imperial Corsair speed values are bugged: https://issues.frontierstore.net/issue-detail/74601
The weight of a fully equipped ship (combat build) is under that mass. Until the ship is heavier than 945t the speed will remain constant.
Not that I don't think the ship should be faster / more agile, but that is a different dog...
Ignore the Legal Issues, an accidental murder or 10...