This isn't true with Kernel Mode Setting anymore. With KMS you do get
enough memory reserved to handle the largest resolution your card can
do, regardless of what was plugged in at the time X launched.
As I mentioned, I don't think the problem is with X not having enough memory for largest resolution, but not reading resolution information when you hot-plug a HDMI connection (or maybe other interface types). If I manually add the modelines using xrandr, it just works.