Is the heirloom even a thing, if you consider that you can hit more than one target with mirrors most of the time?
In our last Mannoroth-Kill Blademaster did nearly 8.5% of my Damage. I doubt that the heirloom trinket with way less agility can make up for this. Would be interested in logs though.
Used it with Mirror for our first attempts on him last night. 27% uptime and 3.52% of damage done over 27 pulls, not too exciting. Logs are private sadly.