I think it's the formula from safehouse that's the error. I did some fooling around with my lvl 20 rogue (on a "real" server) to test it.
Unbuffed I have a STR of 119. Then I buffed myself with my 54 shammy and got up to a STR of 253 (my BS skill is only 70 due to PL'ing).
My max backstab unbuffed was 60 with a dmg 9 piercer (haven't really twinked my rogue

), buffed I had a BS of 89.
According to the safehouse-formula, the maximum dmg increase from the extra 134 str would be :
(skill / 25) * (STR /100)
Which would be (70/25)*(134/100) = 3,752
Unless I suck at calculating, this seems like a way too low dmg-bonus for adding 134 to STR.
I don't know how safehouse got that formula, but I think someone should be able to come up with a better one.
Ztaar
-- Nunc ille est magicus --