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My reason for obsessing over this matter is that the db I've inherited (8.1.7.4 on HPUX 11i 64bit using raw logical volumes) is not utilising async IO (the driver isn't in the kernel).
Given, however, that db_file_sequential_read represents the most significant response time component for any query that I've ever looked at on the system I'm worried that async IO will actually degrade performance
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You only get the best from RAW when the POSIX implementation AIO is configured with HP OS. How ever, since AIO is not configured in your system, you can do some test by increasing DBWR processes (DBWR_IO_SLAVES).

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.........but there could be some factor that led to less blocks being read from cache during the bench marking of raw with async IO.
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The best throughput will be the lowest the denominator of disk controller through put and disk throughput. What is the maxmum number of physical reads that the disk subsystem achieved?
Today's disk subsystem technology has improved a lot. Even with raw devices, EMC disk arrays can be configured in such a way that any controller can access any device, and I have seen a true load balancing now-a-days.

Tamil