Every race is EXTREMELY close, with single digit raw DPS damage differences. If you care about performance on specifically Beast type bosses, the most impactful racials will by far be
Big Game Hunter and
Beast Slaying from
Dwarf and
Troll respectively.
Alliance
Night Elf
Night Elf and
Human are the best 2 races for damage, the only reason
Night Elf is ahead is because of the slight chance the
Starshards priest specific racial gets buffed or tuned to become better than
Mind Flay. Other than that,
Elune's Light grants crit which synergizes with
Shadowforms crit damage boost.
Human
Human is right next to night elf with a passive crit chance increase from
Ruthless works well with the crit damage boost from
Shadowform.
Human also gets 10% bonus spirit from
The Human Spirit which will help prevent you from running out of mana with
Meditation. Also very useful in PvP with
Will to Survive.
Gnome
Gnome gets
Eureka! but only affects non-periodic abilities for 8s, so DoT's like
Shadow Word: Pain and
Devouring Plague are not affected, making it fall behind
Human and
Night Elf slightly.
Expansive Mind grants some extra max mana but hopefully you never run out anyway with the mana discounts from
Shadowform and the massive mana regeneration from
Meditation.
Dwarf
Dwarf only gains
Big Game Hunter, will be slightly worse than other races in general but on Beast bosses will be the best racial by far, an entire 5% extra damage. It is unknown how many Beast bosses there will actually be so this race is placed last.
Horde
Undead
Undead's only damage racial is
Touch of the Grave, while not extremely powerful the Horde race options are limited and it is still better than the small Haste cooldown from
Troll. You also gain some extra mana from
Dark Sacrifice but will probably end up not being needed at endgame.
Troll
Troll gains
Berserking, being very low impact as mentioned in the Stat Priority section with DoT's not gaining anything from Haste and an already strict casting rotation making the casting speed not useful.
Beast Slaying will by far be the most impactful racial on Beast bosses for 5% extra damage, but it is unknown how much use this will get.