please forgive all the shorthand... I have NOT done alot of testing... this is a GENERAL overview of my thoughts on rotations. Legendary and gear WILL change your experience.
Burn/ opening is still very similar....
[with AMP, Slip, RoP, Res, Chrono, ErsN, OP ]
abx4, MOA, RoP, AP, AM (s), PoM AB, AB, AMs...
Sometimes after OP i am at 100% mana...(especially with good AM procs)
so I start burning with ABs till i have 2 AMs, then RoP mini- burn the AMs, ... down to 0% mana.... Barrage, EVO (slip movement with chrono speed) ... then Ang.Acendance on their face

now.... with AP/Evo on CD:
its all about managing mana for a minute... barrage!!!
I think everyone should be able to ABx4, (AMs) barrage for ST...
more casts = more chances to proc, crit, trickets, etc..
when OP is starting to come back you can widdle down to 30% mana, pop everything on your second AP and burn everything. evo ...
I dont know if this helps at all... on heroic Ursoc a 2:14 minute fight I barraged 5 times ( after the 1 minute lust on pull burn)... on Mythic Odyn a 7:18 min fight 17 times... mainly to get a speed boost during the rune/protection/add phase.
TLDR: mana problems? barrage! xD