Cancelling drift of a gyro by mounting?
Maybe it depends on your gyro, but as far as I know the drift issue is not related to the measurement hardware on the Z-axis. Mounting the gyro on its side will have exactly the same drift as the standard mount. The drift associated with the yaw is typically larger because it is normally corrected for using the magnetometer, which has a much less reliable vector than an accelerometer. Meaning that the acceleration due to gravity, the down vector letting you know that the gyro is not moving on pitch or roll is much more consistent a measurement than the Earth's magnetic field. Especially if taking measurements indoors. There are plenty of things that create magnetic fields on order with the Earth's, none that match its gravity (assuming surface of the planet measurements).

One of the most effective drift cancelling methods I've used for a full IMU is zero angular velocity update. Very basically, if there is any time you know the device is actually stationary, you zero out the gyro. Depending on your application this might be after a footfall for a foot mounted device or any other recurring resting state. Works for the accelerometer too, though the zero velocity state measures acceleration due to gravity, giving your updated gravity offset.

I believe that Invensense devices have some pretty fancy data fusion on chip that should help sort all of this out but, to get very good results, you'll probably still have to use a Kalman filter.