The difficulty spike is way too high for only an extra 500ish xp. There are way to many Elites. If there was a 5-10% increase to the normal amount of elites, and double or triple the horde encounters, that would be great.
As it stand Endless Horde feels like the difficulty is raised 2 additional levels, but without any of the payoff that would incentivize players to seek it out. Just play a harder difficulty and get even more xp and make more marks.
So as it stands I think that the current Endless Horde should stick around, but be called something different so people know what they are getting themselves into. It should also have double the xp for the mission.
Endless Horde on Malice (difficulty 3) is definitely not as difficult as Sedition (difficulty 4). Which difficulty do you play on?
Double XP seems a bit much, perhaps 50% more would be adequate, but it can’t surpass the extra XP from increasing difficulty, at least from Malice and onward.