Not that I doubt this, but was this in Summer Knight? Because if so I think a lot of that early stuff is open to interpretation. Jim has soft-retconned more than a few things from that time after all. Not to mention, in the early books a much bigger deal was made of mortal magic and that they had the big advantage. For example, Red Court vampires weren't supposed to have sorcerers/wizards that could pull off the same feats as humans outside the Never never. Which clearly has been proven otherwise, you only have to read Changes to see this isn't the case.
Naagloshii can take mortal magic, yet it doesn't give them mortal magic. They take the energy, a big boost of it probably, but because they arn't mortal they don't get mortal advantages. At least, based on the rules of the series so far. We haven't seen what happens to a Naagloshii after it take a mortal's magic. The Nightmare only seemed to get a bit of a boost from Harry when it ate a bunch of his chi. It didn't seem to get special magical advantages.
So I wouldn't think Winter or Summer would get any special benefit from a Wizard being sacrificed on the tables. But who knows? Jim might retcon things one way or another.