Tuning modified IRIS model in SITL

Hi, I modified the parameters in the IRIS model’s SDF files to match the parameters of a custom larger drone of about 15 kgs. I got the values of the parameters from the CAD model of the custom drone.
For this, I changed the base link mass and inertias, respective prop positions, rotor mass, area and inertias. I also changed the max rpm parameter in the iris_with_ardupilot/model.sdf file.
I followed the tuning instructions as per - Tuning — Copter documentation
However, the drone is quite unstable and there is a lot of sideways translational movement on commanding takeoff through QGC.
I also tried giving only throttle input using the virtual joystick in QGC, but that led to the drone drifting in the XY plane along with rising up.
Is this the correct way to go about changing the model parameters? Are there any potential errors in the process I followed, or am I missing something important?