The moment I got on the PTR I wanted to see if the 10% Penalty was functional or not. I assumed that if it were actually implemented, and not just a tooltip change, it may be more likely or less likely, depending on the results, to be a realistic change.
I filled out my spec (leaving out only TG), then began wacking a target dummy without any mods. I noticed 1300-1500 swings and 2700-3000 crits without TG.
I then specced into TG and was assuming I would see a noticeable drop. No noticeable drop occurred, in fact it seemed as if I was hitting harder just having specced into it. 1400-1650 swings and 2800-3100 crits? (huh??)
To make sure it wasn't RNG I spent the entire time spam right clicking on my buffs to prevent Flurry, Procs, and FotFF from stacking. I decided to get recount onto the PTR for a better test:
http://i4.photobucket.com/albums/y15...sting/noTG.jpg
http://i4.photobucket.com/albums/y15...ting/yesTG.jpg
100,000 Damage is a small sample so I know it is still subject to RNG. Procs and buffs of all types were removed and did not have any effect on the testing at least.
From the test it appeared, up until the end when I got a miss streak, to actually be a 10% damage increase.
I've been playing around with it a little bit more and I feel confident enough to say the tooltip is either:
A) Wrong, and in fact supposed to be a 10% increase.
or
B) Only a tooltip, and the 10% decrease has not been implemented.
Try it yourself. It looks like good news so far. No nerfs and possibly a buff?
Again, still too small a sample to tell.