This is a problem for the operating system to solve, and such solutions out there are already good enough that PostgreSQL has little reason to try and innovate in this area. I routinely see seq scan throughput double on Linux just by tweaking read-ahead from the tiny defaults to a sane value.
I spent some time going through the various tuning docs on the wiki whie bringing some new hardware up and I can't remember seeing any discussion of tweaking read-ahead at all in the normal performance-tuning references. Do you have any documentation of the kinds of tweaking you have done and its effects on different types of workloads?
