Okay, just got off the phone with a GMAC Customer Service representative (my initial email was not answered). She said that
it is actually possible to score up to a 60 in the verbal and quantitative sections,
but so far no test taker has ever performed above a 51 for either section. Obviously, though, many have earned a perfect 6.0 on the AWA. As a side note, the representative seemed somewhat surprised at my question; she actually asked me, "Who told you that?"
Also, the verbal and quant sections are not graded on the same weighted scale, so we can't simply add them together to arrive at a number. I guess this means that a 51Q does not necessarily equal a 51V.
She also said something about using the midpoint or mid-something to determine a test taker's score, but I couldn't follow. She said it so quickly and I blanked during that moment. Sorry about that.
This was very interesting to me, and I thank you for asking this question. Hopefully, you find it worthwhile as well.