Pretty sure thats not how the whole piece count thing works.I wish there'd be an option where you could merge all the pieces into one. I mean really one piece that you can't ungroup anymore. That would solve the piececount struggle.
But I wouldn't want to trade it for the normal grouped-custom-item-thingy we have now. I would want it as a seperate option.
Merging 100 of the little rocks into one piece with a grouping tool isnt magically reduce their impact on performance, not in any meaningfull way i think.
1 piece isnt any set value, it drasticly varies depenfing on how many polygons it has, textures, if its animated and so on, all of these would probably stay the same between 100 rocks and a "100 rocks but combiend into one piece".
I think its comparabele to a Lego set, how many pieces it includes is in theory barely anything to go by.
Because a set that has 100x 4x2 bricks has several hundred times the material cost than one that has 100x of the little 1x1 round plates.