Some tasks won't perform all that much better on SAN than DAS(even spinning disks). I have no idea how else to describe/categorize what's what, but that's been my observation. One of them being a certain database application(not the famous ones ). But there are a couple of things to keep in mind:
1. Did you check the latency? I'd collect a perfstat and have NetApp tell you if volume and LUN latency counters are reporting high numbers. With your setup, at that number, both counters should be pretty low. If that's the case, then the delay is somewhere beyond your storage.
2. Did you tweak thread count/parallelism? I don't know about the tool you're using, but these stress test tools usually let you play with thread count/block size/data size/file count/random-ness/read% vs write%.
3. At the risk of sounding like a NetApp apologist(I'm a customer), could some of this be on your tool? A few of our HA pairs are like yours(32xx with SATA aggrs) and a Microsoft stress tool for Exchange 2010 POC pumped out 800+MB/s without the latency hitting 10ms and the bottleneck was that the links were 2x 4Gbps. I think these numbers you and I get can depend on the tools and their settings.