Would you run your fileservers on a publically accessible network? If not, then the above attack would require the attackers have access to your private, internal network and at that point it could be argued that you've already lost.

Of course, it's good to design defense-in-depth, so that you could survive a hit on your internal network as well. But I'm not sure how much sleep I'd lose over the possibility of a DDOS attack against a private network.

On the other hand, if your servers are on a private network, I'm not sure why you'd use SFS over something like NFS.