I know not exactly the same hardware wise, but theory wise similar. On a MacbookPro 2.5 last gen. and using the standard PCIe Sonnet esata card, with (2) 3tb Graid3. Im cutting in proreshq 1080p 23.98.
I'm getting a minimum of 2, but have gotten up to 4 RT streams, depending on the effects and the amount of audio streams in the cut.
My Kona system test has me at speeds of at minimum 90mb/s range with 25% capacity left on the drives. The sonnet pro card I borrowed for a while was giving me 140-150mb/s. Not too shabby.