Darren,
I think I may know what your issue is. I ran into this same issue at a client while implementing a Nearstore VTL 700 and performing restore tests on a FAS3040c. We saw backup speeds in excess of 100+ MB/s. However, when we did restores, we only saw speeds of 4-6MB/s. It turned out that the system was being affected by BUG 230194 - http://now.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=230194 . This is easy to test. If you are working with a clustered system, temporarily disable clustering (cf disable) and run your restore test. If you the performance picks up as expected, then you are affected by this. According to NOW, this was first fixed in 7.2.2 however, at the time, my client was running 7.2.3P4 and it was still present. We upgraded them to 7.2.3P8 (will be upgrading them to 7.3.1 when ready) at the time and the problem was gone.
Regards, Andre M. Clark