For now, we'll talk about the cycles proposed by songster:
a) "Normal" 4+r/4+n/(4+n) cycling - i.e. always use 4+ finishers, and Envenom as many times as possible between ruptures.
"Tight" degenerate cycles: Mutilate / Rupture / Mutilate / Envenom, paying no attention to combo points. This cycle never drops S'n'D and never clips Envenom buffs, but may have an issue overwriting Ruptures and wasting energy on low-point finishers.
c) "Middle" degenerate cycles: Mutilatex2 / Rupture / Mutilate / Envenom or Mutilate / Rupture / Mutilatex2 / Envenom, once again paying no attention to combo points. This cycle will sometimes waste a combo point before the Rupture, and sometimes waste energy on a low-point Envenom. It is very unlikely to drop S'n'D, but may have an issue overwriting Ruptures.
d) "Extended" degenerate cycles: Mutilatex2 / Rupture / Mutilatex2 / Envenom, paying no attention to combo points. This cycle will never waste energy on low poin finishers, but will waste combo points from good SF/Ruthlessness procs. It also runs a risk of dropping S'n'D.
For simplification in the rest of this post, I will refer to these as Normal, mNmR, mNmmR, and mmNmmR.
My first thought was based on existing tools, the best way to do a comparison would be to use Simulationcraft, as I figured it would be far easier to modify its action queue in order to mimic these various cycles. Of course, this assumes that there is a certain amount of veracity to the Simulationcraft rogue modeling. And while it may not yet be 100% accurate, it does seem to be sufficiently peer-reviewed and mature that it's worth at least checking it out.
So, my initial attempt at modeling the four cycles showed that mNmR was only ~1% less than Normal, mmNmmR was ~2% less than Normal, and mNmmR was the biggest difference, about ~5% less than Normal.
Looking more closely at the numbers, it seems like the reason mNmR is so close to the Normal cycle is quite simply, you perform substantially more Envenoms (~37% more!). And as a result the Envenom uptime is actually higher for mNmR. And even though the energy regen and DPE of each of those Envenoms are lower than in the normal cycle, you actually do enough additional finishers such that your energy gain from Relentless Strikes is almost identical.
At first I thought this might be an error in the way I set something up, but after further thought I suppose it makes sense. If you assume ~45% of the time you will be at <4 CP when doing a Mutilate after your finisher, then it's really not that hard to see that if you decide to do a finisher there instead of another Mutilate, your number of finishers will go up. Basically mNmR compresses the cycle and although there are some damage losses, the damage gains from the compressed cycle seem to make up most of the ground.
However, there's one big question mark regarding the mNmR cycle. Namely, what do you do if you can't overwrite your Rupture due to AP procs, which will happen multiple times even in a short fight. Do you wait until Rupture fades before reapplying it? Or should you just do another Envenom? To be honest, I'm not entirely sure how Simulationcraft is handling that case, so that would have some effect on the numbers. One other thing I can't quite figure out is why mmNmmR has the highest Envenom uptime of all the builds.