You won't see much faster read performance, since you are close to the 1 gb limit already.
You could try jumbo frames - I'm not a huge fan of them myself, but they can improve speeds with some clients. You'll need to ensure that you pick the right MTU for your clients and the switches between the clients and the NAS.
if your volume is old, you could try a factory reset. There are some settings changes that Netgear has made over the past releases that only are applied to new volumes.
But I think you are likely correct in thinking encryption is the cause of the slower write speed. You might also want to consider why you use drive encryption. What benefits do you see? If you leave the key in the NAS there really isn't any security benefit. Someone stealing the NAS+key can do an OS reinstall, and gain full access to the NAS using the default admin password. Plus it doesn't protect from network threats.
If you are CPU bound due to encryption, then you won't see much speed gain with enterprise drives. More RAM could help some if you are running out, but I don't think it will get you to 100 MB/sec write speeds.
