Flexbeta has posted a comparision of SLES9 and Windows 2003 server. The article includes an "out-of-the-box" comparison of Windows and SLES file sharing speeds (using netbench). The results look good for Samba:

With this hardware Windows 2003 Server seems to max out on performance at approximately 30 Clients with a throughput of about 135Mbps, where SLES seems to max out on performance at approximately 60 Clients with a throughput of about 255Mbps. The response time is also about twice as fast on SLES9 than on Win2k3 on the same hardware. So, in theory, you can handle twice as many clients on the same hardware using SLES9 compared to using Windows 2003 Server.

Also:

Novell's SLES9 pretty much more than doubles the performance of Microsoft's Windows 2003 Server on the exact same hardware in both categories. This is very, very impressive, and shows the strengths of both Samba and the Linux kernel [...]

For more, head over to Flexbeta and read the full report.