As was stated before, hitting high notes involves a relaxed voice.
The natural tendency when singing in your comfort zone is to push all the air out and belt the notes, mainly because it's your favorite part of the range of your voice. You cannot do this when you hit the higher notes out of your range. Although you should try to keep your diaphram firm as when singing any note (this helps the tone of your voice), you should take a delicate approach when hitting the higher notes. I tend to keep my mouth and throat open only enough to let the words and the note out, and let my throat try as naturally as possible to work in the high range. The result is that you will hit the notes, but they will not be as loud.
To sound consistent, you will need to work the mic. Move in close to the mic during the high notes (and low notes) at the limits of your range, and move the mic back when singing normally.
Cy