How are CIFS and SMB related to each other

What is the OS of your client and server? One thing that can make a difference is to update your servers to windows 2008 and your clients to Vista. When you do this you get to take advantage of SMB2 which is less chatty, has larger buffers, and can do multiple things in a single request making it less sensitive to latency.

In my experience the kernel mode server out performed samba with my clients. If performance is your number one concern, skip samba. That said, there are a number of limitations to the Solaris kernel-mode SMB/CIFS server, most notably:

• Only runs in the global zone. Samba can run in multiple isolated zones and/or the global zone simultaneously.
• Sharing happens at the filesystem level, not the directory level. So for new shares, zfs create pool/fs a new zfs filesystem, copy data over and share it (instead of sharing an existing directory)
• No following symlinks in shares, unless they are on the same filesystem.
• No child mounts. If you share a filesystem, sub filesystems are not shared. (e.g. two filesystems, pool/fs and pool/fs/subfs. If you share pool/fs, you won't be able to access the contents of pool/fs/subfs without sharing it separately. It'll show up as a directory via SMB, but will be inaccessible.
• None of the Domain controller/AD Master, WINS Server and other niceties of Samba

Of course doesn't do cross-protocol locking (a file locked via SMB is also locked via NFS when nbmand=on is set with the in-kernel server) and doesn't do VSS integration so snapshots show up in the Windows 'Previous Versions' tab in the properties window.

If you can live with the limitations of the kernel mode server and you don't need zone-level isolation, I think it's the way to go. If you're a heavy Linux/Samba user now and like some of it's unique features, feel free to stick with it. Also of note, if you're running SmartOS the choice has been made for you, they make it nearly impossible to run stuff in the global zone (with good reason) so you'll have to use OmniOS, OpenIndiana or Oracle Solaris if you hate Samba.