Is Mac-book Pro Mid 2015 up-gradable? Does the newest model of Mac-book pro 15 inch 256 GB (mid 2015)
support Samsung's 950 pro NVM-E M.2 storage (512 GB)? 
Will it also improve  sequential Read &amp; Write speed of the system as
the later form of storage is NVM-Express ?


NOTE: I tried to look it up over google &amp; SE but I couldn't find any mention of MBP Mid 2015.
No. Apple uses special blade SSDs. There are some aftermarket blades made/sold by manufacturers such as OWC. Please pay attention to get the correct blade. They differ from the blades for the MBP 2012.