Try adding this to the aircraft.cfg to see if it helps:

[GPWS]
max_warning_height         =  1000
sink_rate_fpm              = -1500
excessive_sink_rate_fpm    = -2000
climbout_sink_rate_fpm     = -100
flap_and_gear_sink_rate_fpm= -100

Just so you know, these are copied from the CS757 aircraft.cfg where they are *commented out. The one of interest to you is max_warning_height=1000 and to a lesser extent, flap_and_gear_sink_rate_fpm=-100. Change it to your preference.

*From the FSX SDK help. Use the marks "//" to add a comment, or comment out a line you do not wish to be processed.

Hope this helps.

Mark