Any improvements in the boosting process in the last few years?

[hyphnos]

That's interesting about RAF going to 90. Did you link your accounts after or before 6.2? I have an active link but haven't done anything post 85 since 6.2 dropped.

EDIT: just tested summon friend on the older linkages and an 85 could summon another 85 which would normally be disabled so this seems to confirm it works to 90.
Also at OP:

Boosting in dungeons is best but it it is much better and depends on how fast you can move through them. My last batch of 4 took a bit over 6 /played (not super rushed and some time idle) They put most of the XP in dungeon "quests" that repeat, ie a big hit of XP at the end after you kill bosses (plus some big hits for "bonus objectives, particularly numerous in Stratholme). Toons don't have to be super close to kills but they can't always be left at entrance to get credit.

It is worth noting that now if you train ONLY the basic level of riding (the 4 gold one or whatever) when you hit 71 you get everything through basic flying (not Azeroth, Northrend, Pandaria licenses) for no cost.

My alliance path, to give you an idea

get to 11
Shadowfang x 2 with quests on second pass gets you to 23 (warning: you can ENTER Shadowfang at 11, the summon stone there doesn't work on 11s so you have to take one toon up, friend summon the other with your booster or park a lock alt there, afaik this is the only place you hit that issue)
Scarlet Halls x 2 with quest on second pass gets you to roughly 30
Scarlet Monastery x 1 with quests will get you to 33
Scholmance x 1 (no quests) will get you to 37
Stratholme Main gate x4 turning in quests on last run will get you to 53
Stratholme Service Ent x 3 will get you to 57 (turn in quest on final run if it will get you there) (caution it will often BARELY get you 57, so make sure you clear everything)
Hellfire Ramparts x 2 with quests on pass 2 get you to 59
Blood furnace x 2 with quests on pass 2 gets you to 65
Setthek Halls x 4 will take you to 70 (you can throw in Mana Tombs once with quests for variety if you like) (other two at Auch don't loop on themselves)
Azol Nerub x 1 with quests at 70
Old Kingdom x 4 with quests (2nd or 3rd run) will get you 75
Halls of Lightning x 3-4 will get you to 80
Blackrock Caverns x 1 with quests will typically get you 82
Stonecore to 85 (2.5 to 3.5 passes)

This pathway is set up to minimize moving around - it's not always the optimal dungeon at any given time but for me moving takes more time than running through a dungeon.

85 to 90 either stormstout (leave new toons at door, summon PLer out with them after over) or Jade Serpent as the final hits are actually good without RAF XP and I'm sure insane nice with.