256 was set some time when we were testing a differnt issue. I read that the only drawback is the amunt of time required for recovery, which was tested and it was like 10 seconds for the 256 segments, and higher values mean less disk usage.
Anyway all these parameters should affect the throughput to the data disks, not the WAL, Am I right?
Do your graphs show any correlation between number of WAL segments getting recycled, and disk I/O spikes? Are you logging checkpoints? If so, you could use the checkpoint times to compare against your I/O graphs. I am by no means an expert here, I'm just throwing out ideas (which might already have been suggested).
