I tested the XP gains on testlive at various levels and I am happy to say that hecklers are no longer the best way of levelling In fact, SL itself has been trumped by RK's classic dungeons.
From 60 soldier kill:
~2% side XP gain
Level 56 Exarch, Live server = 4565
Level 56 Exarch, Test server = 9258
Level 80 Nasc Heckler = 10398
From 75 MP kill:
~10% side xp gain
Level 80 Nasc Heckler = 8715
Foremans
Level 60 Heavy cargo haul bot, Test server = 7139
Level 70 Alpha bio construct, Test server = 11490
Level 80 Security Guard, Test server = 17082
Level 80 Security Officer, Test server = 11238
Level 80 Bodyguard, Test server = 22926
Level 100 T.I.M, Test server = 30615
CoH
Level 70 Dark Cenobite, Test server = 11490
Level 72 Ecliper, Test server = 12505
Level 80 Abyss serpent, Test server = 17082
Level 81 Kizzermole, Test server = 17716
Cyborg barracks in Tir
Level 70 Cyborg outside = 7360
Level 72 Cyborg inside = 12505
From 191 Advy kill:
~2% side XP gain
Dark Ruins (teamed)
Level 200 Climbing Creeper = 190370
Level 200 Climbing Creeper = 200389
Smuggler's Den
Level 180 Mantis = 183642
Level 161 Smuggler = 118066
Level 165 Smuggler = 130577
From 220 Shade kill:
Teamed
Level 220 Penumbra Heckler = 302535
Unteamed
Level 220 Penumbra Heckler = 419050
Alappa (Teamed)
Level 220 Malah = 530296
Level 225 Spinetooth = 530805
Level 214 Bleeding Shadow = 406538 (could be alphad with a burst+FA)
While I did not test them myself, apparently the XP in subway pretty much caps for a leveling player, as does xp from cultists as long as they are higher level than you. Pandemonium was giving 1.2 million xp according to another player.
From what I see, subway to 25 -> totw 35 -> foremans 80 -> smuggler's den 125 to 140ish -> IS for 125+ will be a good way for everyone to level. As a side note, you can apparently warp from borealis directly to smugglers den now so be sure to take advantage of it.
Thanks Michi