The orchestra credits for the Soundtrack would imply it wasn't synthesized. (At least those parts)

http://en.wikipedia.org/wiki/Dreamgirls_(soundtrack)#Orchestra

# PIANO: John Beasley, Tim Carmon, Eric Griggs, Greg Phillinganes, Kevin Randolph
# SAXOPHONES: Frederick Fiddmont, Daniel Higgins
# GUITARS: Darrell Crooks, Eric Jackson, Michael Thompson, Randy Spendlove
# TRUMPETS: Wayne Bergeron, Gary Grant, Jerry Hey
# BASS: Nathan East, James Johnson, Harvey Mason, Jr.
# TROMBONES: Steve Holtman, William Reichenbach
# DRUMS: Glendon Campbell, Gordon Campbell, Ricky Lawson, Harvey Mason, Jr., Harvey Mason, Sr., Anthony Moore
# HARP: Gayle Levant
# PERCUSSION: Harvey Mason, Sr., Harvey Mason, Jr.
# KEYBOARDS: Harvey Mason, Jr., Kevin Randolph, Randy Spendlove, Damon Thomas
# CELLI: Larry Corbett, Steve Erdody, Suzie Katamaya, Daniel Smith, Jennifer Kuhn, Vanessa Freebairn-Smith, Rudolph Stein
# BACKGROUND VOCALS: Melissa Bereal, Richard Bowers, Raven Dillard, Natalie Ganther, Nicole Ganther, Larry Greene, Camile Grigsby, Cassandra Grigsby, Chara Hammond, Eric King, Erica King, Kalia Rafa, Kevin Shannon, Nicole Thrash, Robert Thrash
# VIOLAS: Robert Becker, Denyse Buffum, Andrew Duckles, John Hayhurst, David Stenske, Shalini Vijayan, Kristin Wilkinson
# VIOLINS: Roberto Cani, Darius Campo, Susan Chatman, Daphne Chen, Mario Deleon, Armen Garabedian, Berj Garabedian, Endre Granat, Alan Grunfeld, Julian Hallmark, Neil Hammond, Geraldo Hilera, Sharon Jackson, Peter Kent, Songa Lee-Kitto, Natalia Leggett, Demitri Leivici, Cynthia Moussas, Alyssa Park, Sara Parkins, Michele Richards, Anatoly Rosinsky, Haim Shtrum, Tereza Stanislav, Josefena Vergera, John Wittenberg, Kenneth Yerke

In order to avoid Gordy's and Ross's lawyers, the stage production was scored to sound deliberately NOT like Motown material. Unlike the film - where they put the action where the real-life events and people were, after this was suggested by David Geffen.

So while the stage score was using Philly soul and disco/R&B as its template (heavy strings beds a la Salsoul etc), the film production team rearranged the score to better match the Motown stomp, gospel inflections.. very much the Funk Brothers sound...

As it was effectively tracking the Supremes' and Motown's story with "Florence Ballard" as the main arc...

General consensus among musicians (esp bassists) is that with Nathan East on the soundtrack.. they nailed it..

Have a look at the Wiki entries...
http://en.wikipedia.org/wiki/Dreamgirls_%28film%29

Especially the section on the music changes.