The ship's total mass must be close to the mass of the water it displaces. Less mass spent on the hull means more mass spent on the reactor and steam plant.
The displacement is based on the volume of the sub not the mass.
Basically the Alfa was a small and light sub with probably the "biggest" reactor in terms of power output for it's size.
What you got is a sub with half the displacement of an LA class submarine with a reactor with equal power output which meant that you could strap on some really powerful engines.
So half the displacement with about the same power capabilities means pretty much that you can double your speed.
mass = volume * density. Water has a (nearly) fixed density. So for any given displacement, less mass allocated to the hull means more mass allocated elsewhere.
Yes but the mass doesn't effect the volume of water displaced if you put in a hollow 10 cubic meter steel cube into water or a solid 10 cubic meter steel cube they'll both still displace the same amount of water which is 10 cubic meters.
BTW the density of the sub is also for the most part pretty much equal to the density of water.
The only times where it's going to be either greater or lesser than water is when it's doing a static dive or fast ascent.
When the sub does it's initial dive and takes on water into it's buoyancy tanks it becomes about the average density of salt sea water once that is achieved it can use it's diving planes (foreplanes) to adjust it's depth.
