I'm not much of a theorycrafter, but I'm going to have a go at this, and hopefully others can correct and build on it.
Inspired by the
report of Lrac (from the compendium thread), I did some testing with an elemental shaman, and my conclusion, same as Lrac, is that when the demon crits, the DP buff is calculated from your current spell power minus your current DP buff, rather than your current spell power bonus category buff (including totems).
That is, totems boost DP, but (obviously) DP does not feed off itself. The only buff that does not contribute to DP is DP itself (excluding bugs).
However, due to the non-stacking nature of totems and DP, when your demon crits, if DP is already up, the calculation arrives at a reduced value, because it subtracts the DP buff, and you don't have a totem buff anymore because DP replaces it. So you will notice that if you have a totem, when your demon first crits, your spell power will go up (assuming you have enough sp to beat the totem and get the DP buff), but when your demon crits again before DP fades, your spell power will go down by 10% of the totem (or possibly less if your DP buff without the totem is less than the totem, in which case the totem buff will replace the reduced DP buff).
I played with a /cancelaura Demonic Pact macro, and
with this macro, I was able to keep the higher DP buff on my shaman friend always, at the cost to myself of the loss of the DP buff, dropping back to the totem.
Those were my observations, and I'd appreciate if someone with more experience in all this could verify my conclusions, since the rest of this theory depends on it.
With this macro'd to your main spells, you should be able to maintain an extra 28sp to the rest of the raid if you have an elemental shaman using ToW, at the cost to yourself of the difference between your unboosted DP and the totem. That loss will become larger as your spell power increases, but I think not large enough to outweigh the 28sp to the rest of the raid at currently attainable gear levels.
This thread has been using the metric of 8 casters multiplied by an average 1.5 dps increase to convert SP to rDPS. Using that maths, and assuming that one of the casters is the DP lock, the ToW boosted DP would be worth an additional (7 * 1.5 * 28) = 294 (call this bDPrDPS, for boosted DP raid DPS gain, to be used in calculations further down).
The cost to the DP lock is (ubDP - 280) * SPScaleFactor, where ubDP is the sp of your unboosted DP (ie. Your DP calculated without a totem). For the DP lock, the value of spell power seems to be less than 1.5, judging by the simcraft thread, which has sp scale factors for 3/52/16 of 0.99. I will use 1.0 for SPScaleFactor (is this a reasonable figure?). So to cancel out the benefit to the other casters, solve the formula:
bDPrDPS = (ubDP - 280) * 1.0
294 = (ubDP - 280) * 1.0
294 / 1.0 = ubDP - 280
(294 / 1.0) + 280 = ubDP
ubDP = 574
So 5740 sp (without totem and excluding DP bugs such as with demonic aegis and life tap glyph). At some more reasonable gear levels (again, without totems or demonic aegis or LT glyph), the net gain of this trick in terms of raid dps is:
ubDP = 3.0k SP: bDPrDPS - (ubDP - 280) * 1.0 = 294 - ((300 - 280) * 1.0) = 274
ubDP = 3.1k SP: bDPrDPS - (ubDP - 280) * 1.0 = 294 - ((310 - 280) * 1.0) = 264
ubDP = 3.2k SP: bDPrDPS - (ubDP - 280) * 1.0 = 294 - ((320 - 280) * 1.0) = 254
ubDP = 3.3k SP: bDPrDPS - (ubDP - 280) * 1.0 = 294 - ((330 - 280) * 1.0) = 244
ubDP = 3.4k SP: bDPrDPS - (ubDP - 280) * 1.0 = 294 - ((340 - 280) * 1.0) = 234
On the shamans side, this requires the use of ToW, which comes at the cost of any potential dps gain from the use of other damage totems, but does cover the small DP downtime, and covers the absence of a ret pally. If your elemental shaman can drop a damaging totem instead of ToW, then this trick may be nullified.
Also, if you do not have an elemental shaman, the numbers from flametongue, even talented, significantly reduce the value of this trick. With a talented 165 flametongue:
bDPrDPS = (7 * 1.5 * 16.5) = 173.25
bDPrDPS = (ubDP - 165) * SPScaleFactor
173.25 = (ubDP - 165) * 1.0
173.25 / 1.0 = ubDP - 165
(173.25 / 1.0) + 165 = ubDP
ubDP = 338.25
Or approximately 3.4k sp without totem and DP bugs to negate the raid dps gain from the extra 16.5 sp on DP with the personal dps loss. With an untalented 144 flametongue:
bDPrDPS = (7 * 1.5 * 14.4) = 151.2
bDPrDPS = (ubDP - 144) * SPScaleFactor
151.2 = (ubDP - 144) * 1.0
151.2 / 1.0 = ubDP - 144
(151.2 / 1.0) + 144 = ubDP
ubDP = 295.2
Or about 3k sp without totems and bugs - very achievable, and beyond that point, the trick becomes a raid dps loss.
The real gains depend, obviously, as with all this discussion, on your raid composition. And many of the numbers are vague. I haven't factored in DP uptime into these calculations, but since with this trick, DP downtime is covered by ToW, the difference will be small (DPDowntime * (DP - 280)). Also, there is, again, no mention of the gains of the extra 28 spell power for healing.
There used to be an addon,
TankBuddy, that had the ability to remove BoP and salv on tanks immediately as soon as they were applied, I believe (never used it myself). It hasn't been updated for a long time and I don't know if it is still possible for an addon to do that, but if it is, that could help make sure you maintain the totem boosted DP.
Just don't forget to remove the /cancelaura macro if your elemental shaman is away
So, how did I go? Any errors or oversights? How useful in practice is this likely to be?