Looking through the forum here, but also over at ardupilot.com and the Facebook group, one can see many folks who have either complained of poor flight performance of their IRIS or had crashes due to power loss towards the end of a flight.
Taking the data from IRIS and running it through eCalc.ch, one can very quickly see that IRIS is too heavy and underpowered to fly safely at the weight of IRIS + Gimbal + Tall Legs.
Therefore my recommendation: Don't fly your IRIS with Gimbal and long legs! It's not safe and you run a very high risk of crashing it!
Flying with the short legs and GoPro but without Gimbal is OK. Although that's not what I purchased it for.
Have a look at the table below. The "Hover Throttle (Normal)" is the amount of throttle you need to give at the beginning of the flight to keep IRIS hovering. The "Hover Throttle (Low)" is the amount of throttle you need to give when the battery is down to 25% capacity. That's about when you should land. What unfortunately becomes very clear: IRIS is not suitable to be flown with that weight at any realistic air temperature or altitude. Required hover throttle is always more than 70% (making it a "very underpowered copter"; http://copter.ardupilot.com/wiki/ac_throttlemid/). But in all cases you even go beyond 80% required throttle at the end of the flight. You can't control a quadcopter anymore at that level.
If you want to reproduce these results yourself with http://www.ecalc.ch/xcoptercalc.php, here are the values used:
# of Rotors: 4
Model Weight: 1750g (incl. Drive)
Elevation: see table above
Air Temperature: see table above
Battery Cell: Custom
Cell Capacity: 3500 mAh
Resistance: 0.0052 Ohm
C-Rate: 30C cont, 40C max
Controller: max 20A
Motor Manufacturer: RCTimer A2830-12 (850)
Propeller: APC SlowFly SF
Or use this link.
What hover throttle do you require at what air temperature and field elevation? Post some logs along with air temperature of that day and field elevation.