Okay, good point! So let's add Combat Potency into it. My understanding is that we have a 20% chance to proc MG with a 1.6 weapon, and therefore a 32.5% chance to proc MG with a 2.6 weapon.
So the chance to proc MG and have that MG proc CP is 0.2238 * 0.325 = 0.0727. So a 7.2% chance to generate 15 energy when using Eviscerate. As this means you'll proc it once every 13.9 Eviscerates we can conclude using 15/13.9 that you will earn an average of 1.08 energy in addition to the damage dealt.
So on 3 targets, the difference between Tempest and Evis is 880.5 damage in Tempest's favour. 1.08 energy is definitely not worth the loss of 880.5 damage.