the rest of the things you mention negate better efficiency without A LOT of specific parts and A LOT of time tuning.
the spring kit will help you achieve all of those things just in different doses.
the harder springs will increase efficiency with higher pressure, make it more unnecessary to hold the pump handle forward in order to get good velocity, and make the stroke STIFFER (not smoother, only a different pump kit, delrin bolt, and sear/hammer polishing will improve smoothness)
I like a stiffer pump stroke. it's snappier.
the softer springs will lower your pressure and efficiency per ball but will also make the gun quieter, the pump stroke lighter, and increase your potential autotriggering speed slightly. if you were running semi it'd also allow you to operate the pneu's at a lower pressure which would make the gun less likely to chop a ball in half.
the process is basically trial and error after an educated guess. put your springs in, put the ivg into the middle, lower your input pressure to almost nothing, gas up, and increase pressure until the velocity stops increasing. if it isn't right start over.
...'cause bandaid's stuck on me!