MAP Sensor

Generally speaking; at what point is it worth considering upgrading the MAP sensor/housing/etc. when increasing engine hp? 20%>, 30%> 50%> ?

The MAP (Manifold Absolute Pressure) sensor is independent of horsepower, you should use one with adequate resolution for the manifold pressure you will be measuring. If you are increasing horsepower by boosting, then you need to have one that can handle the maximum boost pressure you will see.

Perhaps you meant MAF (Mass Air-Flow) since you said housing. You will need to upgrade it when it's a restriction, or can't measure the airflow.