Like the OP I’ve experienced similar issues with X on some of my systems. Earlier versions of NetBSD (and Xorg) worked a bit better on configuring some of my setups. When Xorg was modified to try and reduce the need for an xorg.conf file things really messed up for me, but as Xorg has evolved over the last couple of years it has been getting better in this respect.
Although I haven’t looked into the Xorg code, my feeling it is trying to use a combination of decoding the EDID and some set of internal tables. On my systems I’ve seen cases where setups that were in the EDID for my monitor didn’t show up during configuration and some cases where the opposite is true. I also see similar disconnects with what appear to be configurations listed in the x.log files which seem to be coming from internal tables, like the ones the OP is addressing with his modifications. There also seem to be some issues with things like blanking times and video placement on the monitor that I’ve had to adjust with MODELINES in my xorg.conf files that again appear to be addressed in the proposed modifications. Finally, for some reason I usually see fewer problems in getting my monitors configured when using various versions of Linux than I do with NetBSD when running on the same HW. I don’t know if these anomalies are in the Xorg configurations since both seem to use the same versions, or if the anomalies are a result of differences in the lower level drivers. In most cases I’ve been able to work around these by copying some of the MODELINES coming out of Linux and putting them in an xorg.conf file on my NetBSD systems.
