I'm not so sure about that. With my WarpEngine 4040 with Samba 2.2 on an SFS volume I could get a max of about 263k/s to an Athlon 2800 Windows XP box. That speed increased when I moved to the CyberStorm MKIII 060, though I don't have a concrete number to back that up right now. It's also much more peppy responding to Windows Networking.
Of course a lot of that difference could be simply in the SCSI interface and hard drive. On the 4040 I used its narrow SCSI with a 50-pin IBM 18GB DeskStar. On the MKIII I'm using a its wide SCSI with a 68-pin 18GB IBM.
How much native PPC code are you running when doing your transfers? I wonder if that changes anything. Even so, I expect FTP transfers to be faster than Windows Networking... Windows Networking does not bandwidth efficient at all. Also, are you talk 250kBYTES/s in your serial transfers? That's like 2.5Mbits/s, and I'd hazard to say you're not getting that over serial.
