For the record, there is a bug with 4T12 and Primal Madness, as explained by Yawning here.

Essentially, with 4T12, Primal Madness is lost after 2 seconds of using a 5pt finisher while under the effects of Berserk or Tiger's Fury, which is mimicking the behavior of Berserk extension, without any of the benefits.

Facts:
1. If you start berserk at full energy and end at 0 energy, you do more damage than if you start at less than full energy and end at 0 energy. Because you have more energy to spend.
2. 5cp FB still has higher DPE than shred during berserk. (Check out Mew)
3. FB costs 37.5 energy and shred costs 20 energy during berserk. Assuming full hit/expertise, and a rotation of 4 shreds 1 FB, in 25 seconds the total energy spent is 587.5. You would need to have an average energy regen rate of 19.5/s to be not able to finish at 0 energy (assuming 100 energy at start).
4. Of all raid buffs only heroism and dark intent increase energy regen. Heroism gives 3/s, DI 0.3/s.
5. H The Hungerer (15s) & H Matrix Restablizer (30s) gives 13.5% haste each, totaling 2.2/s energy regen. 2000 haste from gear gives 15.6% haste, equivalent to 1.56/s energy regen.
6. There is no way you'll ever hit 19.5/s energy regen during berserk, i.e. you'll always hit 0 energy during berserk, unless you get an OOC streak.
7. Starting at X energy (X<100) and ending at 0 energy is completely equivalent to starting at 100 energy and ending at 100-X energy, from an energy POV.

Conclusion: no reason whatsoever to not start berserk at full energy. With 4pT12 it is all the more not to be considered.

Please kindly point out anything incorrect, thanks.

Edit:OOC

For the dpe comparison of shred and berserk can you please supply some numbers? The only reason I chose to source from simcraft data instead of mew data was because mew data did not provide the details required; such as damage per bite.
Based on stats still available on simcraft, I listed how and why I derived a full energy Ferocious bite is less dpe than shred.
The simcraft numbers crunched previously can be found previously posted on this forum post #48.

Since there seems to be some dispute as to the dpe conversions of FB and Shred during berserk, im going to pull samples from my WoL of last week Balrok and re-calc:
World of Logs - Real Time Raid Analysis
FB Crit avg
63609.3 / 37.5 = 1696.25 crit fb dpe during berserk
FB non crit avg
26561.8 / 37.5 = 708.31 noncrti fb dpe during berserk
Shred Crit avg
36568.8 / 20 = 1828.44 crit shred dpe during berserk
Shred noncrit avg
17801.9 / 20 = 890.1 noncrit shred dpe during berserk

NOTE: These are actual results from a fight that took place, not simmed or theorized figures.

Now that we have established outside of 4pt12 shredspam > fb spam for dpe, I re-state my previous posts: To make the most of your berserk you play to your limiting resource; if limited by energy shred spam for the maximum dpe; if limited by time 5point fb spam for maximum dps.

Also when I crunched the original post about being able to shred spam while maintaining energy, I factored the energy gains of OOC, which I found to dramatically increase your energy/second; as well as dodges. No stat set I have seen actually suggest that expertise > crit, so it is unlikely that you would actually be expertise capped.

You can't accurately determine ferocious bite damage by taking the average over the course of a fight because its damage varies with the amount of energy it consumed. (This applies both to actual parses and to simulationcraft's summary.) You are coming up with a DPE figure that's too small because you are averaging low-energy and high-energy ferocious bites into the damage figure while assuming maximum energy consumption in every case.

Even with your numbers, taking into account the +25% crit modifier for FB, FB has higher dpe. Your FB crit is much higher than FB hit, so it can't be very accurate.

Numbers from Mew with 391 gear template and 0/1 berserk talent:
shred DPE: 698
FB DPE: 1131
which translates into ~1400 DPE for shred and 1500 DPE for FB during berserk.

With a second look ferocious bite does crit much more frequently than shred, so instead of calculating shred and FB independently here are their hit+crit averages then divided by appropriate energy costs to find dpe:

FB Crit*#of crits + hit*of hits / total fbs / 37.5 = avg dpe of FB
63609.3*9+26561.8*5 / 14 / 37.5 = 1343.41 Bite dpe during berserk
*note this assumes every bite was full energy which was not the case

Shred crit*#of crits + hit*of hits / total shreds / 20 = avg dpe of shred
36568.8*44+17801.9*56 / 100 / 20 = 1302.97 Shred dpe during berserk

Thx for the tip on FB's crit byjiang, I overlooked it before. Mentioning again that Tol'Vir is applied to 60% of the bites involved in this calc but only 20% of the shreds, but also considering not all bites were full bites as computed, I would guess that indeed FB would probally lead by a small margin.

Note also that the difference here of 41dpe translates into ~5*37.5*41 or 7687.5 total damage difference, ie 1 melee crit. It is far more important to not energy cap, once again suggesting to start one's berserk ~ 100/120 or 80/100 energy.

Does anyone else experience a .5 second delay between using FB and the extra energy cost being taken? The least amount of energy I can spend on a FB is ~30 total outside of berserk or ~18 inside of berserk. Even with a minimum energy cost of 18, small bites crush the dpe of berserked shred.

they hotfixed the 4pT12 to work ''properly'' with Primal Madness. Will edit with link when I get back on my computer (currently phone-posting)

EDIT: August 29

Classes
Druid
The Feral druid 4-piece tier-12 set bonus should now interact properly with Primal Madness and Euphoria.

Source: Patch 4.2 Hotfixes - World of Warcraft

Not sure why it includes Euphoria though :s

Ferocious bite consumes up to 25 additional energy, not 35 (despite the tooltip).

I am sure FB consumes up to 35 additional energy,I tested。

You should retest that. It consumes up to 25 additional energy, despite the tooltip. Not 35.

EDIT: VVV Or just simply FB when at 100 energy and see that it takes you to 5X, not 4X. It's really obvious.

I test again, I had concluded was wrong。 It indeed consumes up to 25 additional energy。