Dell Perc 5/i Performance issues

Hi,

I've bought a Dell Perc 5/i because I couldn't make the onboard marvell 
88sx7042 work with 8.0/8.1 or current, but as lucky as I am, the best I 
can do with 4x1.5tb samsung in raid5 is 60MB/s writes and 90MB/s reads, 
with bbu/write-back/adaptive-read-ahead.

I was expecting at least twice of that, and I'm not sure what can I do 
to get that speed. (I've read man 7 tuning with no success)

As far as I know this controller should be as fast as on other systems. 
(Freebsd.org mx1 has one of these cards.)

I'm hoping somebody on the list reads this and helps because I can't 
afford to buy another card.

Thanks in advance,
-zsozso