A FlexRAID pool does not make your drivers significantly faster. It can perform a bit of pre-fetching, but in the overall transfer speed that doesn't matter. There is no striping in FlexRAID, so it can't read one file from multiple discs for speed up (like a RAID 5/6 could)
My guess is that the IO patterns in such benchmarks are either hitting some kind of cache, or FlexRAID detects the benchmark attempt and artificially speeds it up.
If you want a real world test and have a SSD in that system, then run some read/write operations by copying files around from/to the ssd (to ensure the source/destination is fast enough), or if you have a mingw/cygwin environment with the "dd" tool, thats usually also good for simple read/write speed tests.