At risk of being ultra-geeky, this is an article which discusses the effect of brew pressure and espresso:
http://pubs.acs.org/doi/abs/10.1021/jf0206623

Specifically, at 11 atm, the concentrations of 2-methylbutanal, 3-methylbutanal, and 2-ethyl-3,5-dimethylpyrazine are higher. These compounds are generally responsible for bitterness and astringency.

At 7 and 9 atm, methanethiol and propanal concentrations are higher. At 9 atm, these compounds tend to be slightly higher compared with espresso prepared at 7 atm. Methanethiol is responsible for the "freshness" perception, and propanal is correlated with "fruitiness" generally.

Well, How cool is that!

I reading this book I got from my local library by interlibrary loan:

Espresso Coffee: the Science of Quality 2d ed. Ed: Andrea Illy and Rinantonia viani

Check It Out!

many thanks again!

And the technical paper that Henry so geekily referenced MAY also be available from your local university technical library - Boston, MA does have its fair share of those.

Jim Schulman (another_Jim) referenced this paper in some earlier post and I did manage to get a hard copy from the U of Md chemical engineering library. In addition, if you do a search on "pressure profiling", you'll see where Greg Scace (GregS) mentioned that taste went "downhill" at pressures above 9.0 bar.