About _LOG_SIMULTANEOUS_COPIES:
Oracle say, that this parameter set = CPU_COUNT * 2 automaticaly
(little note: CPU_COUNT from init.ora, but not from equipment of a real box)

If u have >2 CPU, u can increase value of this parameter up to CPU_COUNT * 2 + 2
without any perfomance degradation.

About _LGWR_ASYNC_IO and _LGWR_IO_SLAVES:
1) U should check, is ur OS support async io operation, if yes then this is way to
increase activity of disk operations in lgwr.
2) U should set _LGWR_IO_SLAVES = 1 !!! if u going to change _LGWR_ASYNC to TRUE.