Can someone break down or link a site that explains what the effects of stacking various chord tones are? I'm only looking at guitar atm, but I'd imagine it has the same general effect on piano, etc.

For example, what generally happens when you add another 3 and 5 to a major chord, without adding another root? Playing around in FL studio, I've inverted a lot of chords and stacked a few chord tones to try and achieve different voicings, but I've never really noticed any patterns (yet, at least).

It's so much easier for my ear to pick up on things when I know what to look for -- hence this topic. I'm gonna guess the effect really depends on which notes you stack and/or invert, as well as the pitch of the added notes... but are there any "generalities", so to speak?

Or, if there aren't, am I on the right track when I assume stacking, say, fifths, will make it more prominent and give the chord greater tension? Or that adding another third will... well, do something? Lol, frankly I'm not sure what the mediant does aside from define major/minor tonality.
The third in a triad is typically what makes the chord sound "happy" or "sad". If you voice it in the highest voice the chord tends to sound the "happiest" or "saddest". Fifths don't really add much as they are very consonant with the root. Adding more roots thirds or fifths won't effect the tonality at all. They can be useful for adding interest to a repetitive chord progression though. Inversions and different voicings typically occur for voice leading reasons, which makes the chords flow smoothly (or if you want them to sound very rough, you could go against the conventions I guess). Typically you shouldn't be too concerned with the number of each chord tone that is voiced, as long as each important tone is voiced atleast once.
In general the lowest note gives the root of the chord and often defines what the chord is.

The higher notes give the "sound" of a chord, so putting a major 3rd as the highest note will make things sound more "major" where as putting it low down will sound less major. I would say the lowest one and highest two/three notes in a chord really define how it sounds to me. But it does depend on the register (the actual highest and lowest pitch of the chord)