Hmmm, redo_buffer. You can have a bottle neck when writting to the REDO LOGS. Now, Oracle suggests that you probably won't get much of a boast in perfmorance in your REDO_BUFFER by increasing it > 1 MB. Even though.. I increased mine to 4 MB, and performce has been better. I'd look to see what device your REDO_LOGS are on, since writing to redo logs is performed sequentially, there is NO benefit is placing them on any drive that is stripped.

I also agree with roobaron what suggested, definatley add an additional 2 logs, make this change before fiddleing with ou LOG_BUFFER sizing.

Cheers,