I wonder if the IMU brick 2 can be used as a slope indicator for grade control. How accurate is it ?

Getting a concrete accuracy value is not easy, as the IMU Brick uses multiple sensors and a data fusion algorithm to generate it's measurements. We only specify a measurement resolution of 0.0625°, but no measurement accuracy. You can try to find more information in the sensor's datasheet.

If you only want to measure the slope, you could also use an Accelerometer Bricklet, but then have to calculate the slope from the acceleration values yourself.