@hmlow ,
I know your post was a while ago.. But I wanted to add some detail.
Going to 15Hz is going to reduce the maximum exposure (duration of emitted IR signal) to about 1/3 of the maximum which would be for 5Hz. This is due to the fact that maximum power output remains the same and increasing FPS means less power is emitted during each frame. Longer exposure / IR pulse means the sensor can achieve higher signal to noise ratio and pick up returns at longer distances.
So that would make sense that you may see less phantom obstacles at 15hz compared to 5hz, if these obstacles are indeed coming from multi-path returns (weaker / shorter IR signal has less chance of being picked up by the sensor)