I appreciate the replies and testing info. You make some good points re: SAS in thinking longer term. Hopefully the enclosure(s) will last a long time and that means faster now (even if I don't utilize it right away) would be better. I'll definitely have to think about it more.
It would likely be awhile before I would need to expand if I stuck to my current ideas. I could go up to at least a 10TB volume (5 x 2TB disks) before I would need to think about starting to swap in larger drives. The only thing I think that could cause storage needs like that is 4k video, and since I just went 1080p a year ago, I don't think I'll be looking at 4k within the next 5 years (assuming it even becomes a commercial reality in that period of time). Handbrake works wonders on BluRays, reducing the typical movie down to a file size around 4GB with excellent results.