Windows 2000 and SAPDB 7.4, data disk advice, RAID

Hi,
 
On Windows 2000....
 
have 4 36GB 15,000 RPM SCSI hard drives for the main storage.  Two channel
disk controller, put two drives on each channel.  64-bit PCI LSI/Mylex 352
controller.  For first round - I configured these 4 drives as  RAID 0+1
yielding 72GB of usable space.  Also formatted the partition with NTFS using
a 8K block size.
Log is on a dedicated 7200 RPM IDE drive with no other purpose (also on a
dedicated IDE controller channel).
 
Observations:
Watching kernel.exe with performance monitor, I/O performance on the main
data array seems to be about twice (2x) what a single 7200 RPM IDE drive on
dedicated channel was doing before.   Given the 2x spin rate and 2x drives
(raid 0), I expected a little more improvement (3x to 4s instead of only
2x).
 
Any thoughts or suggestions?  Is NTFS @ 8K block size ideal for SAPDB, or am
I overlooking something?
 
Obviously I could put the 4 drives all in a RAID 0 and get more
performance... which is an option as we have custom replication between our
two SAPDB servers.... but I am undecided if this gain is worth the potential
downtime/recovery time.
 
Thank you for any input.
 
  Stephen Gutknecht
  currently in Long Beach, California USA