This is all interesting stuff, I'm also looking to do more realistic simulations just having got to deep in it yet.
What you'd have to do is work out the math to calculate the capable payload of the drone. If you know the number of motors (4) for iris, thrust, and drone weight you can calculate this according to http://www.rcdronegood.com/formula-quadcopter-weight-calculator-thrust/.
In the iris's SDF file I linked to before you can get the mass of the components tallying it up it comes to 1.535 kg however listed here https://www.adafruit.com/product/1546 it says 1.282kg so not sure why the discrepancy. My next suggestion would be to look at the motor model in Tools/sitl_gazebo/src/gazebo_motor_model.cpp to try and see what thrust the simulated motor is capable. In the real world the props you have would play a big role so I'm not sure if this is expressed in this model or in the gazebo aerodynamics plugin.
This should give you a starting point, I'd be interested to find out what you find, keep us updated!