Secondly Metamagic or metamagic changing feats always specify what effect they have on the slots, even if it is none. For instance some feats in such cases do say:
"The spell slot level is not affected." Or close to it.
Benefit: Once per day, you can apply the effect of the Quicken Spell feat to any spell you cast without increasing the level of the spell or specially preparing it ahead of time. You can still use Quicken Spell normally.
"Metashadow Feats function identically to Metamagic feats in all ways not specifically contradicted herein,"
Wait... you want to knock down the Shadowcaster's already-crappy power?
The metashadow feats sidebar could really be interpreted either way. Metamagic feats invariably spell out exactly what effect they have on a spell's level and they have unlimited usage. Metashadow feats, on the other hand, have no slot adjustment listed, but are limited by the day (thus making them more akin to the Sudden Metamagic feats, none of which have a slot adjustment).
I understand where you see the "function identically to metamagic feats" clause as leading to a slot adjustment, but you should also consider the implications of further reducing the power of an already-weak class. The Shadowcaster is still quite limited in their abilities and Metashadow feats are one of the few ways to make them a little less crappy, if there is no Mystery Level Adjustment. If you do increase the level for the 1/day use of Empower, you are essentially slapping down the Shadowcaster just for kicks.
Since after reading the responses you've gotten, people seem still a little fuzzy, I say you and your friend settle this like real men.
Roll up level 20 characters, have a 3rd person serve as DM judge, and duke it out. Whoever lives, wins the argument.
This is how all disputes should be handled.
I didn't say it was fair, I'm saying it doesn't make sense to me based on the way it's written.
If you'll read earlier, you'll notice that I acknowledge the flaws of the class.