Maxtor 80gb slow on asus p2b-d ? (going to use it in raid5 config)

i'm going to build a raid5 config with 6*maxtor 80gb
mainbord is an asus p2b-d (with dual 450 and 512mb ram)
and 2 addon ultra66 promise controllers

i did some tests using hdparm (4.1)
and i get these result :

(hda and hdc are on the onboard controller)

/dev/hda:
 Timing buffered disk reads:  64 MB in  4.94 seconds = 12.96 MB/sec

/dev/hdc:
 Timing buffered disk reads:  64 MB in  4.94 seconds = 12.96 MB/sec

/dev/hde:
 Timing buffered disk reads:  64 MB in  2.26 seconds = 28.32 MB/sec

/dev/hdg:
 Timing buffered disk reads:  64 MB in  2.25 seconds = 28.44 MB/sec

/dev/hdi:
 Timing buffered disk reads:  64 MB in  2.25 seconds = 28.44 MB/sec

/dev/hdk:
 Timing buffered disk reads:  64 MB in  2.25 seconds = 28.44 MB/sec

kernel reports :
hda: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(33)
hdc: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(33)
hde: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(66)
hdg: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(66)
hdi: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(66)
hdk: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=158816/16/63,
UDMA(66)

and i use hdparm -m16 -c1 -d1 -a8  on all hd's
so why do i only see 13mb/sec on the ultra33 controller
(i thought 28mb/sec would be possible on the ultra33 controller)

i also made a raid0 on the 6 disks
and this gives me :

/dev/md0:
 Timing buffered disk reads:  64 MB in  2.13 seconds = 30.05 MB/sec