Well, with GPWS it's clear - it's a dumb thing that only reacts on your radio altitude, configuration and speed. As for EGPWS - it's a different beast which uses terrain database with terrain elevation measured from MSL. Whatever your atimeter setting, it thinks it's QNH, and gets upset when you are actually on a perfect glideslope at a more or less elevated airport. So, use QNH and do your math. Disabling EGPWS is a bad idea, especially in IMC.