I think the people calculating a buff within the 8-10% range are just looking at their logs and simplistically applying a 25% buff to their Cobra Shot damage and a 125% buff to their Barbed Shot damage in order to get the 'new' damage values. This is wrong for two reasons:
1. It is erroneously applying a 125% buff to Feeding Frenzy damage, which was not buffed. This will cause the calculated value to be overstated.
2. It neglects the fact that Barbed Shot will now ignore armour. This will, by sheer luck, somewhat mediate the error made in #1, but not by enough, so the overall value will tend to still be overstated (assuming they had the Feeding Frenzy trait).
For example the following is the rank 3 mythic Grong parse:
In the parse:
6.97% of the damage is from Cobra Shot
3.70% of the damage is from Barbed Shot
(This person also had 1 stack of Feeding Frenzy)
The erroneous method
Simply applying a 25% buff to Cobra Shot and 125% buff to Barbed Shot results in:
6.97%*25% + 3.70%*125% = 6.37% buff
This is wrong because a 125% buff has been applied to the Feeding Frenzy damage component of Barbed Shot, and additionally it has completely ignored the fact that Barbed Shot will now ignore armour.
The correct method
We need to split the Barbed Shot damage into the baseline damage and the Feeding Frenzy damage, assuming 55% of Barbed Shot damage comes from Feeding Frenzy then:
6.97% of the damage is from Cobra Shot
45%*3.70% = 1.67% of the damage is from Barbed Shot
55%*3.70% = 2.04% of the damage is from Feeding Frenzy
To account for Barbed Shot now ignoring armour we need to remove the 0.7 armour reduction factor that has been applied to Barbed Shot (by dividing by 0.7):
6.97%*25% + 1.67%/0.7*125% = 4.72% buff
(EDIT: I made a mistake here, so the above value is wrong, thanks for andreasels for pointing that out; the correct value is below)
After factoring both the armour ignoring and the 125% buff, the new Barbed Shot damage becomes 1.67%/0.7*2.25, thus the total increase (or size of the buff) will be:
6.97%*25% + 1.67%/0.7*225% - 1.67% = 5.44% buff
These numbers are interesting to me since my theorising wouldn't have had any of the non-normalised stat weights going down since nothing was nerfed with this change. Yes the relative value of Mastery should drop, but the absolute DPS increase from Mastery shouldn't. I do wonder how much of an impact simple random variation in the sims contributed to the differences you saw.
My predictions for the non-normalised stat weights would have been:
No Change: Mastery
Slight buff: Agi, Versatility
Slightly bigger buff: Haste, Crit
Even bigger buff: Weapon DPS
EDIT: Now that I think about it, I don't think I've seen Barbed Shot crit, so as your crit rating goes up, theoretically Barbed Shot damage should become proportionately smaller than the rest of the abilities (which would do proportionately more damage as crit increases), but this is offset by the fact that crit increases the Wild Call chance, allowing more Barbed Shots to be cast. Similarly, even though haste does increase how often Wild Call procs happen (due to more auto attacks), this increase is exactly in proportion to an increase in how often the other abilities get cast, so it wouldn't actually increase the proportion of damage done by Feeding Frenzy compared to the damage done by the other abilities.
So perhaps neither haste nor crit should scale any better after all. I still don't see how they would scale any worse though, so I'm still going to chalk up any of those decreases to random variation .