And you'd be surprised how many people think MB/s and Mb/s are the same thing. Obviously you know the difference.
My mistake on the "writing directly to the cache drive" statement. Again, sorry I can't be of more help than to say that writing to a "cache enabled" share on my server I can saturate GigE.
What are the specs of your system?
Supermicro SC846 case with Tyan S7025 with dual xeon 5650's and 32gig, 40+TB of mixed WD red and HGST deskstar NAS, using HGST 7200rpm 6TB for parity drive and 500Gig Samsung 850 EVO for cache. Drives are connected to LSI2008 controllers, Mellanox Connectx-2 10Gig network.