Question on the "Empower" metamagic feat

[Eldritch_Lord]

Yes, but it is a variable numeric that would change between a level 5 caster and a level 8 caster. If Wally the level 5 wizard cast Magic Missile, he would get 3 missiles. If Sally the level 7 Sorceress cast Magic Missile, she would cast 7 of them. Hence, the numeric is variable depending on your caster level.

Similarly, if Wally casts something that lasts 1/round level, his spell would last for 5 rounds, while Sally's would last for 7. Again, the numeric is variable depending on level.

Here's the thing, though--you're not preparing the spell as a 5th and 8th level caster. When you prepare the spell, as Joe 5th-Level Caster, the number of missiles, range, duration, etc. are fixed. They don't change at all after you prepare the spell, and since you prepare a fireball with Empower Spell rather than learning an empowered fireball, for all intents and purposes those numbers are not variable, they're constants.

While I agree with your interpretation, is there any official clarification that means a variable numeric applies to effects that involve a roll of the dice?

Most official clarifications aren't worth much ("Customer Service: 100% official, 50% accurate!"), but as Runestar said, he quoted the 3.0 FAQ, and the ruling is supported there.