I've found sometimes better performance using the OEM's drivers over Symantec's.  They are often updated more often and may provide additional performance.

Now since it's smaller files, that sounds about right, for the slower backup vs larger files.  This is normal behavior.  Can you get more speed, perhaps?  It depends on a lot of factors, and the biggest bottleneck could just be NTFS, file fragmentation, and poor disk tuning.

You want to benchmark your NAS and see what kind of throughput you can get when doing an FTP transfer, and disk to disk transfer.  May want to give HDTach a shot for some low level benchmarks as well.

For the drive itself, try a larger block size.

Those settings should be OK.  You could try setting the buffer to 128 or 512K if you want, as long as you have sufficient RAM to handle things

As tervia-boy says, though, lots of small files will always backup slower than a few large files totalling the same backup size.  This is due to the overhead involved in accessing and opening the smaller file as opposed to just streaming the larger once accessed