If you set a DTD, and configure the driver to ignore EDID, it should be at the resolution you set.
A couple of things could be going on here:
1. You are NOT configuring the driver to ignore the EDID properly and the monitor's EDID has some odd characteristics that cause the driver to pick the wrong mode- likely the display suggests the "best" mode which is not really the best mode.
2. The driver IS setting the desired resolution but you are looking at the on-screen display of the monitor for the resolution which is not always accurate. If you use the IEGD GUI, the mode shown there IS the mode the display is operating at despite what the monitor may think.
You might take a look at that and see what might be going on.