After reading about this on and off for past 6 months this is the conclusion I have come to.
Every post I've read on anything stacking after testing seems to add up to Nada.(1-2% increase)
Really makes you wonder why there's a feat called "Blinding Speed"!? lol (if it does zippo to the 15% Melee you already have)
LMK if I'm missing something cause early on I swore I read 40-60% was possible, but thinking that was nerf'd out of game before I started playing.