Our current systems were built a few years ago so I think there are probably better choices for the specific components now.
One of the things we also did, which in retrospect was a mistake, was to spec a CPU with a lot of cores. The idea at the time was that we would run some VMs directly on the storage server for tasks that needed super fast data access. However, once we got everything running it didn't make sense to actually do that. Since the storage servers are just storage servers we would have been better off using CPU's with fewer cores and higher clock speeds.
At the end of the day, I don't think the exact hardware is actually that critical for TrueNAS. I think getting a decent understanding of ZFS and the tradeoffs different configurations entail is more important for getting good performance. There are a few different ways to configure it "right", but there are also ways to configure it that will have awful performance and cause a lot of pain.
Check out Lawrence Systems
, he's done a bunch of videos on ZFS and TrueNAS over the years and is a great resource if your interested in learning about this stuff.