Mercury Elite Pro Dual mini only getting 250MB/s read and write
These are HDDs not SSDs inside? 250 MB/sec is good speed for a RAID 0 on two hard disk drives. The 860 MB number is probably calculated using maximum speeds reading data that is already in the hard drive's disk cache. Meaning you'll his that number for a millisecond, but once you start reading from the platter, you're limited to the maximum read speed of a physical disk.... times two. 

I've had good lock with Mediasonic USB3 drive towers. I have their 8 bay but they make 4 bay too. four 2 TB drives in a RAID 0 (hardware or software) should give you much faster throughput.