I have a guruplug server plus and tried to use a 1TB USB drive that is NTFS formatted.
Disk read write was abysmal from a Vista laptop 100 kb/sec: likely a combination of wireless
connection, SAMBA and ntfs-3g driver.
Ntfs-3g performance on a Sheeva variation would not be too good. Ntfs-3g runs through FUSE in userspace, so there is a a lot of data moving from kernelspace to userspace and back to kernelspace. Then add on the fact that Samba is running in userspace so the data move into userspace once again and then back to kernel space to go out the wire. The little arm processor just can't handle that very effectively. Linus (not very delicately) said FUSE is a toy. I don't entirely agree, but with respect to performance he's absolutely right.
There would be a HUGE difference in performance moving to any native linux filesystem.
USB2 is going to cap at about 25M/sec. If you need faster than that you have to use e-SATA.
An ideal situation would be to use e-SATA, a linux native filesystem and nfs (personally I prefer nfs4). You'd have very few kernelspace to userspace transitions.