[Avid-L2] Disk Speed limited to 300 Mbs on dock that supports Sata III?

I've got a pair of Seagate IronWolf SSDs that are Sata III and spec says they can get approx 550 Mbps in Sata III.  The dock(s) are connected to a Sonnet Allegro PCIe card with 4 USB 3.0 connections.  I can only seem to get around 300 MB/sec per individual drive but when I raid 0 them I still only get around 300 MB/sec.  I had thought given the vintage of my docks they were all SATA II.  I went to Fry's and got the dual dock because it said it supports 1.5, 3.0 & 6.0G Sata drives.  I took that to mean it the dock slots are 6G Sata III.  When I test the individual slots on the dual dock I only get the 300MB/sec.  This leads me to believe that the dock is only Sata II slots.  Even given that if I raid 0 the two slots I still only get just a little over 300 MB/sec.  I figured it wouldn't double but may get 1.5 times the individual drive speed.  I then split the drive set across two docks both USB 3 to the Sonnet Allegro card.  I get a little better like 333 Mbps.  The Allegro is in my cyclone microsystems expansion chassis hooked up to my mid 2012 MacPro using the 16 lane slot to connect the two.  I know the MacPro is Sata II in the tower but I would have thought if the dual dock is truly 6G Sata III then I should be able to get close to 550 Mbps for an individual SSD . 

So what in the food chain is limiting things to around 300 Mbps regardless of whether I raid or not.  The conversion of USB 3 5 Gbps to Mega Bytes per second is 625MB/sec.  So in theory I should be able to get close to the IronWolf spec of 550 MB/sec shouldn't I.  I know there is only one controller in the Allegro card I have.  I've ordered another to get more USB 3 connections and the new one is the dual controller version.  Maybe when I get that in I'll see if splitting across two USB 3 cards helps.  Am I missing some overall architecture limitation under the hood?  I know sometimes I think in theoretical terms without the experience of practical application.

Someone please bit slap me into a better understanding of what's going on.  ;-)

John Moore Barking Trout Productions Studio City, CA bigfish@pacbell.net