performance improvements from putting tables and indexes on seperate disks?

Are there any benchmarks testing whether there are performance improvements on reads and writes when you separate data and indexes on separate disks?

cases:

1. sqlloader direct path load of a large number of records to a table with 4-5 indexes. The indexes can't be dropped during loads since queries take place during the load.

My understanding is that the write process is serial so separating the data and indexes onto separate disks will not improve performance much. I don't have the disks to test this on right now.

2. queries. mainly oltp type queries. any performance improvements for separating the hard disks that tables and indexes are on?

This is on a SAN, but we can have it configured to give us separate disks and show them to us as separate mount points.

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