How much XP do you miss out on if you don't use Story Companions?
 

What if I wanted to use 100% custom companions? Will I be able to get max level without that Story Companion Quest XP?

Easily max level without story companions.
There is so much XP available that a lot of people install mods to decrease the amount of xp overall.

Thanks Wisdom.

I'd recommend playing with Durance at least once as his backstory plays a big role in the history of Eora. As for other companions, they're really not anything to write home about unfortunately; not only are their stories pretty basic and self-contained, but any banter is rather minimal.

Once you Have your keep, you can take on bounties for a ton of xp points.

I like most of the companions.

As do I. The hard part is choosing which ones to bring along, I wouldn't dream of using custom ones unless going for POTD trial or iron or something. Which won't happen.

Mother's backstory is done really well, so I'd recommend her.