It sounds like a disaster waiting to happen. The latency of NFS is just too high (Even if it is very fast for NFS) for it to be safe for a database.
There are workarounds for the stateless mess of NFS in modern linuxes, but if you accidentally access a NFS-mounted database from multiple AWS instances, you might get into big trouble, especially when the machines in question run different OSes (e.g. Windows and Linux) or OS versions.
It'll never be as fast as local disk but the flexibility it provides is very nice.
Our NetApp can saturate 10GBe without even trying.