Conflicting info between Dell Support & manual - SATA ii Vs SATA iii

I have scoured searching the net with various keywords, and found no solid document about this topic.  So, i am here to confirm  if my  laptop  has  SATA 3 capability.   According to the dell online Service Manual (E6420 lattitude),   this system  ships with a  Sata 2  SSD (which is what i currently have on my laptop).  I have found one reference  in my search  where  DELL support  has clearly  said  that  this system is capable of  SATA  iii  support.   See the links below  for confirmation:

Dell support  forum thread -final post  shows Sata iii  speeds are possible.

Elsewhere, i found people  guessing and running  SATA 3  SSD  on E6420.  The service manual itself  does not say anything about the  speed level (SATA 2 or 3).   Is there any way to confirm that  people are getting SATA 3 speeds on this laptop ?  I know  my  new SATA 3 SSD is backwards compatible with  SATA 2.  My new Samsung  EVO 850 SSD  is capable of SATA 3, and i need to optimize it for higher speeds, if   indeed i can confirm  that this  laptop is capable of  reaching  SATA 3. 

Currently  this is the  speeds i am seeing on my new SATA 3 SSD,  without even running  AHCI.