You have to do a RF System Analysis to find the compression point of the receiver with LNA ON or OFF. The requirements depends by the highest input level requirements and by the level of external blockers.
Probably the first mixer have the highest contribution in this test.
When the LNA is ON, first of all you get compression from the P1dB of the LNA, and second, because in this case the gain before the first mixer is high, the system it will accept a lower signal at antenna connector until will compress the first mixer.
When the LNA is OFF you don’t get compression from the LNA, and the gain before the mixer is low, and the system will accept higher input level at antenna connector before compression of the first mixer.