Still not worth it. That last slot is PCIE 2.0 like the M.2 and it will limit the 750, albeit not as severly as the m.2 slot.
A total waste of money, seeing as PCIE SSD, even at full speed, hardly justify their price premium over SATA in normal desktop usage scenarios(you are not running a heavy database server, compiling and/or transferring huge files all day, are you, OP?)