Considering they actually want players to play together, having a penalty for them to party is counter intuitive.
If anything, they could ADD bonuses for partying, thus making people actually prefer to drop trusts and party with others
I'm all for that and would greatly support this change, but the point I was making is that what really kills EP gain is not the fact you're in a party (as long as the party is up to 6 people, different story for 7+).
There is a penalty which is unknown (check BGwiki) but it's a small penalty, so small that you still gain way more EP by being in a proper party, compared to being solo.
The real penalties are others, like the ones I mentioned.
Mostly the level difference. The more levels a target is above you, the more EP it gives you, and vice-versa.
Which means if you're ML5 and someone in your party is ML30, you will get the EP a ML30 would get, even if you're ML5 and would in theory get more alone.
This can be "solved" by using Level Sync, but even with that, your ML will slowly go up, and as it goes up, the EP per kill you get will go down.
The amount of additional "power" you get by getting one more ML, doesn't sadly compensates the loss of EP per kill.
It's a small bump in power alas, it doesn't really allow you to kill that much faster.