People will hate me for this... and thats ok...
But I always felt it should be like this:

The BEST XP parties should get 25k/hr TOPS... and thats with ideal conditions

Solo should get 10k/hr if they are skilled and no where to XP

If PLing is allowed to live (Which I vote to stop PLing but what can we do)... It should give you more XP than solo.. but not as much as a good XP party... so PLing should never be more than 15-18k/hr....
That way if someone wants to PL you, it is more efficient than solo... but 2 people using an unbalanced, exploitative mechanic shouldn't be able to level faster than a skilled group of 8... not EVER

If you think I am being an elitist ass because I have all classes to 50, then consider this...
On my first class... getting 10k an hour was a gift from God... even in a very skilled group....
Took me over 2 months of many MANY grind parties to get there... it was easily 150+ hours.....

With the pace I proposed....
The BEST XP party could cap a class in 75 hours of leveling

Solo would require 180 hours of leveling
and a PL could get you a 50 in about 100-120 hours