My problem as a former trumpet player is that the key of G isn't higher 
or lower; it depends on the pitch the musician is asked to sing or 
play.
A baritone just wrote me:

A song in G may require a sopano to hit a high C that
might be easier for her to sing if it was transposed
down to say F. (Opera singers would do this all the
time). A song in A could just as easily have a
narrower range, so the singer who would have trouble
with a song in G might have no trouble in a song whose
tonic is a whole note higher.

That said, there are bodies of songs that tend to have
the same range, so a folk or blues singer may find
themselves singing in the same key because all of
their repertoire uses pretty much the same range. It
seldom makes use of difficult high or low notes within
the key. So you hear people say that a certain key is
too high. I tend to sing blues, folk, rock  in G,
which fits my voice and my hands.

Something tells me that two song writers would know this.

On Feb 28, 2005, at 11:33 AM, Honor Conklin wrote:

>   I found a reference to the song in:
>
> Random House  Historical Dictionary of American Slang.  New York:
> Random House, 1994, V. 1, "A-G," p. 851
>
> "up in G" is a reference to the key of G in music and means "doing very
> well" and "at or to a high point"
>

Is there no explanation where they got this?