So far, I've gathered that more mass = less usable jump drive distance from your drive.

And that's quite straightforward and easy to balance.

But hat about speed and maneuverability in combat ? Because it seems that with a big mass, all I need to do is balance it with the right engines to get back my 100 maneuverability.

But do you mean to tell me a light starfigter striped down to minimum mass won't be spining and turning faster than a full imperial star destroyer of a ship, that also hits the 100 maneuverability ? Does a behemoth mass actually makes you sluggier in fight or does it not matter so long that you built it balanced to 100 base point of maneuverability ?