Do you have any additional devices to test this with?
Unfortunately wireless sensitivity can vary even between similar devices due to component variation, environmental factors, antenna design, directionality, etc, so -40dBm may be the physical limit of your current setup, but anecdotally I have certainly seen C7 modules receive signals below -40dBm.
Using a CC2571 instead of an AP2 as the transmitter can get you additional distance as the CC2571 has up to +4dBm output power compared to the AP2.