OK,
Here is a real world scenario to show what kind of dps people are losing with panther on them:
Level 75 Berserker
a 40 delay weapon
100% hasted
Dueling my shaman
No Panther:
With Panther:
As these parses are short the dps values reported by EqCompanion can be ignored (due to variance in hit / miss and the damage range not being accurate over a 10 minute parse, if it was a 10 hour parse we could look at those). The important values to compare which should be accurate over such a short parse are:
Delay:
No Panther: 20 (as expected from a 40 delay weapon with 100% haste)
With Panther: 22
Thats a 10% decrease in dps
Total swings:
No Panther: 463 + 416 = 879
With Panther: 428 + 364 = 792
Thats also about a 10% decrease which backs up the delay difference above