As a summary to an interesting thread from OpenBSD-misc, but that is actually not OpenBSD-specific:
ALWAYS BUY LSI MEGARAID CARDS WITH A BATTERY UNIT
…or you will get very poor performance.
Roy Kim said that with his Intel SRCS28X (LSI Megaraid 300-8X in disguise) adapter, writes bog down at 3 Mb/s, which is terrible.
Marco Peereboom (the OpenBSD I/O adapter wizard) pointed out that LSI megaraid cards will ALWAYS disable write cache whenever there is no battery backed up memory on the card. No exceptions. The only thing you can do is to purchase a battery-backed unit and replace the current DIMM.
That way, you can get about 100 Mb/s with the same hardware.