Back in 2003, I published the results of experimenting with various values of _spin_count to improve throughput on latch congested systems. You can find the original paper here.
The study involved varying spin count on an Oracle 8.1.6 database suffering from heavy cache buffer chains latch contention. The database was running on a Windows 2000 server. The results of this study confirmed that _spin_count can be very effective in improving the throughput of latch contention-bound systems. In the test, throughput almost doubled when spin_count was raised from the default of 2000 to 10000.
I recently repeated these tests for Oracle 11g. This time, the database was experiencing shared pool and library cache latch contention and was running on RHEL 5.0. The results will be published soon in my contention series at ToadWorld. To summarize however, increasing spin_count in 11g was equally effective in reducing latch contention. As _spin_count increased, latch waits reduced and throughput increased, up until CPU saturated, at which point no further improvements were achieved.