sounds loud on the video... I thought piston slap too.
Did you try a fuel system cleaning?... at a shop?... not guaranteed to work, but if your are positive that its the injectors, it couldnt hurt to try and remove some carbon from the top end.
2012 Mustang Cyclone, 2002 Mustang GT, 2004 F150 FX4, 1999 Ranger O/R 4x4, 2007 Harley FXDB