Speed/hull speed. Unless the transom is loaded far beyond design this is generally minimal. Sure, you can eliminate some burbles, but unless the lines are quite long, you can actually increase drag. Increased waterline? Moving from 34' to 36' could gain you 0.23 knots... but you have also added weight but no power. Unless you up-rig at the same time I would not expect to find it.
Speed/multihull. Not so clear. Generally somewhere around 10 knots the wake begins to separate from the transom cleanly; unless you modify the entry too, top end speed doesn't change. Weight and wetted surface rise.
Ride. Cats, which can hobby horse in certain conditions, can often greatly benefit from transom mods, particularly for a specific sailing area. This is the real reason for cats. Would this apply to a mono? Perhaps, but certainly less so.
. Sure, you will gain load carrying capacity. Could make sense for certain designs, not for others. And I love my sugar scoops.
Space. Length does not add useable space, as the beam is fixed and the extension will be sealed.
Doing a good job--one that will not lower the resale value of the boat--is quite an undertaking. Is it worthwhile? The answer depends on the design, the goals, and the owner. I would focus my design on ride and access, if it were me. The only boats I have personal expereince with are Stiletto cats; the stretched cats are smoother but not faster unless other mod are made. Often as not, the whole rig is changed as part of a refit
, so comparing most of the modified boats to stock is apples to oranges.