The maximum result listed for mirror image is list unlike for any other spell. Most of them will say max +10, or max +5 or whatever. Mirror image has a subtle nuance that once you reach a certain level (21 caster level), you'll always get 8 images. This is unlike, say, ray of enfeeblement where the bonus (the +) is limited.
So, do other spells have maximum values? In other words, is 1d4+1 for magic missile have a maximum value of 5, such that it would limit empower as well? If 1d4+1 does not have a maximum of 5, then what do you get when you maximize it? If it does, then how does empower empower it any more than mirror image? Is the explicit language the overriding fact or does implicit language also apply?
IMO, the maximum listed for mirror image is the maximum normally rolled result (as discussed in the maximize feat) and that empower can go beyond the normally rolled result like it does for any other variable spell.