I had assumed that it would just be a flat 150 min/max damage, meaning:
810-1367 = 1088.5 x 1.2 (avg dmg x APS) = 1306.2 dps
If that was the case, emeralds are still better. BUT, here's what actually happens:
150 (+min) x 1.50 (50% weapon damage bonus) = 225 actual +min
300 (+max) x 1.50 (weapon damage bonus) = 450 actual +max
885 - 1667 = 1276 (avg dmg) x 1.2 APS = 1531.2 DPS.
Hooooly crap. So a little over 400 raw weapon dps boost with a Marquise Ruby. I still doubt that it will be a boost in the offhand, but in the mainhand... it can definitely be competitive.
D3rawr already has a function in the tdps calculator that allows you to resocket your weapon with a Marquise Ruby. For me, in my farm gear it's actually a 10k tdps drop going to a Marquise Ruby from just an 80% Emerald, but that's with the slowest 1h weapon and over 50% buffed crit as well. It's a lot closer than I thought it would be, looks like it's going to mostly depend on your gear.