That is true for changes in our Spell Power, Crit and Haste, but not for Mastery. I tested it in-game with a Monk's Elixir. It did 33400 melee hits without the elixir and 34100 melee hits with the elixir. However, if you remove the buff while the totem is up, it won't update its Mastery bonus.
[21:56:59.556] Primal Fire Elemental hits Training Dummy 1 (O: 30130)
[21:57:00.528] Primal Fire Elemental hits Training Dummy 1 (O: 28598)
[21:57:01.462] Primal Fire Elemental hits Training Dummy 1 (O: 30131)
[21:57:02.417] Primal Fire Elemental hits Training Dummy 1 (O: 30130)
[21:57:02.726] Desertus gains Monk's Elixir from Desertus
[21:57:03.453] Primal Fire Elemental hits Training Dummy 1 (O: 30130)
[21:57:04.462] Primal Fire Elemental crits Training Dummy *1* (O: 60262)
[21:57:05.471] Primal Fire Elemental hits Training Dummy 1 (O: 30131)
[21:57:06.461] Primal Fire Elemental hits Training Dummy 1 (O: 30130)
[21:57:07.388] Primal Fire Elemental hits Training Dummy 1 (O: 30131)
[21:57:08.454] Primal Fire Elemental hits Training Dummy 1 (O: 30130)
(source:
http://www.worldoflogs.com/reports/9...D+SWING_DAMAGE