I'm not an expert either, don't get me wrong, but everything I've seen/read about McPhee's range, even from her detractors, points out that McPhee's lower and middle and lower-high registers are really good, particularly due to the ease and fluidity she has switching between registers. These are the parts of her voice that attracted big Hollywood music producers, like David Foster, Tommy Mottola, to gush over her when/after she was on Idol as exceptional etc.
Carrie has power, and you can see that in the videos, but to my ear she doesn't have as good a tone quality or as much fluidity as McPhee does. I'm sure the fact that Hit List is a very Pop/Rock score is a factor as well. The character is a pop-star, and McPhee's voice is wedged in an awkward sensibility between pop and broadway/standards. The role is, in some ways, perfect for her.
It's just a matter of different voice types. Just like Megan and Kat/Ivy and Karen have different voice types and bring/brought different qualities to the Marilyn role. I'm sure there are roles or parts Carrie could sing where she'd be better than McPhee. I don't think this is one of them.