Acquisition sensitivity can be summarized in two  conditions: ephemeris available from any source, or ephemeris must be obtained from the satellite data stream. For conditions where ephemeris is available, we can acquire a satellite down to about -155 dBm.

When we must decode ephemeris, we can still acquire down to -155 dBm, but the signal must be much stronger before we can decode the data. 

That limit is -147 dBm in the latest release of ISM300x 9Sirf III