Ramdisks and hard drives as cache devices

Yes, NFSv3 helps with writing large files. But metadata operations (create, unlink, chmod, mv) still need to be synchronous and some workloads can be very metadata-heavy (untar is a good example, 'make' on a big project tends to delete and create lots of relatively small files too).
So a low-latency cache can definitely improve the performance of an NFS server.