I dunno what special tools filers may have to accelerate this job, but if I were gonna tackle it, knowing only what I know, I'd set up the fastest server I could arrange with the fastest, dedicated interfaces I could, to each Netapp. I'd use rsync[1] to replicate the data initially. Then I'd re-run rsync. I'd keep re-running it until its run-time stopped dropping, and I'd expect that to be after only a few cycles. Finally to make it perfectly synch I'd do one last run with the source readonly, so nobody can make changes out from underneath. I've no idea how long that would take, but I'll betcha it wouldn't be a ReallyLongTime[tm]; rsync is not like rdist, it does not do a separate rpc-like handshake for each file, it streams, very effectively.
I wouldn't be surprised if the Fastest way to do this job were with a fast Sun Ultra or maybe a fast DEC Alpha, using etherchannel or maybe gigabit ether to each of the Netapps.
Even if the Netapp has special local software intended to help out the kind of work you are trying to do, I wouldn't be surprised if something like the above weren't faster; even though they may have tried to put in a special hack, the box is really utterly --- brilliantly --- tuned as a network server, so that's the well-trod route to travel. Lots o' bits been down that road.
-Bennett