I think you're worrying too much about minutae. The fact is it will depend entirely on your particular op mix, so what is optimal for you can only be determined if you run the tests with your real world load. Also, since your load probably changes over time, what is optimal one day won't be optimal the next.
Go with the biggest drives you can get. These are not UNIX or NT servers with RAID arrays that require careful balancing and tuning by you. They are simple appliances. They just WORK. Add more filers for more performance as needed.
Bruce