Stavros,
I'm not sure where you got the information that the 2240 has no read or write cache, but the information is innacurate, maybe you got that from HP.
I can speak with some authority when I say that both the FAS2220 and the FAS2240 have 6GiB of DDR based RAM per controller most of which is used as a unified read-write. Of that memory about 800MiB is battery backed and mirrored to it's partner controller, and is used used to protect uncommitted write operations. After ONTAP operating system requirements are taken into account, on a dual controller system this gives you around 8-10GiB of cache memory for your workloads, which, given ONTAP's architecture, is usually more than enough to extract maximum performance from the relatively small number of disk spindles usually assigned to those controllers.
Depending on your workload, you may get better performance out of a similar number of SATA spindles which are direct attached to a machine vs being attached via some form of network. e.g. single threaded large block sequential workloads usually benefit from the lower latency provided by a PCIe attached raid controller. On the other hand virtualisation workloads are very rarely characterised by that kind of workload. Virtualisation workloads typically involve a large number of simultaneous random reads and writes. As a result the optimisations that are possible from an intelligent controller in a spindle constrained environment usually provide a significant performance benefit.
While I don't have the bandwidth to help you troubleshoot your issue, there are a number of free tools available from NetApp such as OnCommand Performance Manager that will help you isolate where the performance problem might be, and you can always open a performance troubleshooting ticket with NetApp support.
Regards
John Martin
Principal Technologist - NetApp ANZ.