Do they use the same model that the EGPWS database uses?
If so there is no way I would trust it to be anything other than a cloud breaker in an emergency.
the touchdown point is generated by survey on the plates as is the other end so it is reasonably easy to set up the runway in a database. Your altitude would be a be suspect on it though. The rest of the surrounding terrian will be inputed in chucks, What fractal resolution they will have used for that I don't know but I would spuspect that it niot very fine because the amount of data goes through the roof very quickly as you reduce/increase (can never remember which way round it is) the fractal length.
The coming down an ILS is a completely different kettle of fish being calibrated and not subject to data errors never mind all the other features of using GPS which quite a few discount to quickly in my opinion.