Base bonus shot is double damage, so if base damage is 750, 750*2.5 is 1875. The damage is additive. Look at that.
That said, I would still use Quick Draw with Greenskin’s Wickers for bigger on demand burst.
Numerically, the number of procs you get from Weaponmaster alongside Concealed Blunderbuss equals a lot of uptime for the bonus crit chance from BTE plus 4pc, so that’s why you’ll see that pull ahead in terms of raw damage.
Current wording on tooltip:
Half-cost uses of Pistol Shot granted by Sinister Strike now generate 1 additional combo point, and deal 50% additional damage.
Probably should read:
Half-cost uses of Pistol Shot granted by Sinister Strike now generate 1 additional combo point, and deal an additional 50% bonus damage (250% damage).
Otherwise, like I assumed, you would be guessing 50% additional damage times the 200% baseline, or 300% damage.
(You know, or just give Quick Draw the buff instead. I see no issues with that either.)