You seem to still be missing core of the problem ...
If you have Group A who will do just main quest and tiny bit of exploration around ...
And Group B who will search every milimeter of whole world ...
Group B will naturaly come out stronger ...
Sure, if the difference is just "level 4 or 5" then its no biggy ... but im affraid the difference would be much biger.
For one, count out all equipment Group A will miss, since it isnt part of "their quest" ... and for two, the problem with completionist is that if you have route A and route B ... they go both ... so no matter how much you will be spliting XP rewards ... one group will be spliting 200.000XP per act ... but second group will be spliting 300.000-400.000XP per act ... therefore problem stays as it is.
For example, if you would play as Evil character, and would follow only main quest ... your game ended once you massacred grove, since then you are send to Moonrise towers, wich arent there yet, so we dont quite know what rewards are there waiting for us ...
So basicaly ...
If you follow just your road (read as: If you roleplay properly) ... aka if you are Group A player then:
- You either join Halsin, and therefore go to the Moonrise Towers through Underdark ... and never see Mountain Pass, or Shadowlands ...
- Or you join Minthara, who sends you to the Moonrise Towers through Mountain Pass ... and therefore you have little to none reasons to go to the Underdark ...
But if you are completionist ... aka if you are Group B player then:
- You simply explore all those options ... and logicaly you get rewards from every single one of them, no matter how little sense it makes from roleplay perspective that you get there.

Of course there is also possibility that Larian actualy make all our paths to be deadends, and we would therefore travel through whole world anyway and our choices would only set order in wich we will wisit them ... but i hope they dont, since that would be lame as fuck ... even tho there are certain signs of such behaviour (yes, im talking about Nere's broken Lantern) ... so we can expect that just as Minthara send us to her minion, he would for some reason also unable to help us and send us to Nere for change ... we allready know from datamining, that situation in Creche would also be a little more complicated then Lae'zel presumes.

And yes, i admit that in this case my whole point is void, bcs then players from both groups would be going exactly the same route (and therefore gets the same amount of XP and item rewards) ... just in little different order.