OK, time to weigh in on this topic. Don't write off 3% xp as "insignificant", and not worth going into a party for. To get this rolling, I have 4 characters at 50+ and one at 60+, all of which were made when OBT launched. The 60+ character has been in a party with a duo partner every single day. Out of the 50+ characters, 2 were solo the entire time, one was partially solo, partially party play. All characters were in the same guild the entire time, so guild xp isn't a factor.

I'm not here to say, guys look, party play netted me 10 levels because it didn't, and that is unreasonable. I am here to say, I saw significant further progression on party based characters than solo based characters when I hit certain level points. The solo+party character and the pure party character both had to do far less "Urgent" quest than their pure solo counter parts. And before you ask, the pure party character was the first character, so it had to unlock the difficulty on all the roads for the dungeons - meaning some runs were less than optimal for experience gains.

And it's not only the lack of "Urgent" quest that I would say this bonus xp is significant. My pure party character often had epics roll over levels (even at higher levels) whereas the solo characters often would end the current level epic chain around 80-90% experience.

Not to mention the beautiful bonus of clearing more efficiently in a party, which is always a plus.

Get dramatically more items solo than in a full 4 person party

This is just downright wrong. If anything, you get SLIGHTLY more item drops in a party than solo, not dramatically, and even then, we're talking like 1 extra item. Many times when leveling with my duo partner, we had just as many items as solo characters. Remember, when I said that two characters have been in a party this entire time? Yes, that means two characters, in a party, received the exact same loot as a solo character.

That other post you linked mentioned significant more loot from levels 1-20. That may be the case, but who cares? You don't see people spamming megaphones complaining about not having any materials for 1-20 quest. At later levels, this really doesn't hold up. Take chaos stone debris, a fairly hot commodity at the moment. More often than not, when one person got a chaos stone debris to drop, the other got it to drop from that same boss. If one person didn't get the drop, more often than not, the other person didn't get the drop either.

In conclusion, do what you want with this data. It boils down to observations I've had while leveling, and I feel like there's something else we are all not seeing to the advantages/disadvantages of leveling in a party.