I've probably parsed it more recently, but the only thing I can find right now is way back in 2008! See www.brogett.plus.com/rogue_offensive_dps.pdf
Back then it was a 2.7% increase. The spell data claims "Increase Chance to Hit by 5% with All Skills" which actually parsed out at 4.8% fewer misses. So perhaps the maths is correct, just not in the obvious way we think.
If the boost is now 15% in theory then it'd come out as only 5% actual dps gain. Something to parse.
Edit: I can't follow my maths on the 2.7% increase infact. It's correct on dps terms, but I think I just got lucky. 5% fewer misses from a base rate of about 25% miss rate (back then) would work out as 1.7% more hits. I had 1.6% more hits (normalised by time), so I guess I just got lucky on larger hits by random chance: despite 6 and 8 hour parses the difference is so small it's in the noise.