No heavier springs do not give switches a better or really even different sound. Springs are more about the quality of them, regardless of the weighting. Bad springs will sound pingy & sometimes even give scratchy, grinding noises. Although pretty much all springs will have a slight metallic resonance when dry. That’s why most people will use a krytox oil or grease to lube their springs, it will completely eliminate that slight resonance from high quality springs.
Multi stage springs are best described as 2 or 3 springs stacked on top of each other, but they are made from a single wire. The pic below is of a 2 stage spring, a 3 stage spring would have another section, & a single stage spring is just a regular spring.
The deal with multi stage & long springs (14mm is Cherry spec for spring leaf IIRC) is you get more resistance at the beginning of the key press since it will have more force pre-loaded than a shorter spring of the same bottom out weighting if that makes sense. They also will have a more forceful rebound than a shorter spring of the same weighting.
Then there is the opposite of multi stage & long springs, which is progressive & complex springs. With them they start off lighter than a normal spring of the same weighting but more rapidly increase in resistance towards the bottom of the key press. This is accomplished by having the windings tighter on one end of the spring.
So there is some room to subtly change the feel of your switches with different types of springs. I stress subtly though, the difference between any of the different types of springs vs a regular 14mm single stage spring of the same weighting is not gonna be hugely noticeable.
As for your last statement, I’d say most people would consider spring swapping a form of modding the switch. The spring (given it’s a high quality one) will not substantially change the sound or feel of the switch. The materials of the parts, the quality of the molds used to make the parts, & the slider being long pole or not have much more influence over sound & feel than the spring. The weighting & type of spring used is mostly a personal preference.