Re: RAID 5 performance problems

here benchmark from my system with an fasttrak-tx4 (same card jonathan
has) with WD1800BB :

/dev/hde:
 Timing buffer-cache reads:   128 MB in  0.28 seconds =457.14 MB/sec
 Timing buffered disk reads:  64 MB in  1.37 seconds = 46.72 MB/sec

WD1800BB on onboard ultra100 controller :

/dev/hdo:
 Timing buffer-cache reads:   128 MB in  0.28 seconds =457.14 MB/sec
 Timing buffered disk reads:  64 MB in  1.58 seconds = 40.51 MB/sec

WD1800BB on onboard ultra33 controller :

/dev/hda:
 Timing buffer-cache reads:   128 MB in  0.28 seconds =457.14 MB/sec
 Timing buffered disk reads:  64 MB in  2.55 seconds = 25.10 MB/sec

What should I expect from a 8 drive raid 5 on 3ware 7500-8?
I get only 
/dev/sda:
 Timing buffer-cache reads:   128 MB in  0.74 seconds =172.97 MB/sec
 Timing buffered disk reads:  64 MB in  1.55 seconds = 41.29 MB/sec

which seems to be slow compared to the numbers above