new group commit behavior not helping?

Hoping to demonstrate the wonders of our new group commit code, I ran
some benchmarks on the IBM POWER7 machine with synchronous_commit =
on.  But, it didn't come out much better than 9.1.  pgbench, scale
factor 300, median of 3 30-minute test runs, # clients = #threads,
shared_buffers = 8GB, maintenance_work_mem = 1GB, synchronous_commit =
on, checkpoint_segments = 300, checkpoint_timeout = 15min,
checkpoint_completion_target = 0.9, wal_writer_delay = 20ms.  By
number of clients:

master:
01 tps = 118.968446 (including connections establishing)
02 tps = 120.666865 (including connections establishing)
04 tps = 209.624437 (including connections establishing)
08 tps = 377.387029 (including connections establishing)
16 tps = 695.172899 (including connections establishing)
32 tps = 1318.468375 (including connections establishing)

REL9_1_STABLE:
01 tps = 117.037056 (including connections establishing)
02 tps = 119.393871 (including connections establishing)
04 tps = 205.958750 (including connections establishing)
08 tps = 365.464735 (including connections establishing)
16 tps = 673.379394 (including connections establishing)
32 tps = 1101.324865 (including connections establishing)

Is this expected behavior?  Is this not the case where it's supposed
to help?  I thought Peter G. posted results showing a huge improvement
on this kind of workload, and I thought Heikki reproduced them on a
different server, so I'm confused why I can't.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company