A 5cp rupture lasts 28 seconds though (with 2pc) so you have to switch back to reapply poison twice to keep it up. You're gonna need some math if you want to sell that as clearly better. In fact, I'm still skeptical that you end up with a dps gain at all.
I've got the fully updated math above unless anyone wants to apply the probability of # of auto-attacks needed to apply poison each time. The only thing that might not be clear is that I was comparing the cost of applying poison to the energy gain - and then doubled it because it only covered a 14s window. I arrived at a 66.66 DPS gain assuming a 5 CP rupture and the numbers theherecy was using with more realistic AA numbers.
You use the same number of poison procs/melee hits proportional to energy gains etc. Though rupture costs 25 energy regardless of 2cp or 5cp ruptures, hence 5 cp ruptures "recovers" more of that energy.
2cp rupture = 15 energy spent, 60 energy returned if you keep poisons on the target (will need to switch back to reapply once), for a MAXIMUM net gain of 45 energy. Alternatively you could say it's 25 energy spent with 70 energy returned. same thing, really.
5cp rupture = 0 energy spent, for a net gain of 105 energy, but you have to re-apply deadly poison 2 more times. or 25 energy spent with 130 energy returned.
Assuming your values of 12k per melee hit, and 2 hits to proc, you're looking at +45 energy, -96k damage for 1, and +105 energy, -144k damage for option 2. @28.8k mh mut, and 14.4k oh mut damage, your mutilate DPR is 0.785k damage-per-energy. The energy gain in option 1 is therefore worth 35.3k damage and the energy gain in option 2 is worth 82.4k damage.
In both cases the lost 2 autoattack+1DP damage per poison reapplication is more than damage from the extra energy for mutilate. Ideally you'd want to take a weighted average of DPR of all of your energy-costing abilities to get a closer-to-the-mark estimate. Going based on mutilate DPR is admittedly worst-case because the only rogue combo builder with less DPR than mutilate is revealing strike (sinister strike probably also does with haste-focused gemming).