I have a MK II 678GC and MK III 512 and MK III 678Hunter. I like all of them, but in the process of dialing them for myself they received a few mods.
The MK II has a VQ sear, Marvel trigger, VQ rear sight, checkered wood grips and a VQ mag release, rounded to form a nice paddle. I put an Ultradot LT holosight on it and it is super accurate.
The MK IIIs have gotten similar work: a MK II VQ hammer and sear to eliminate the mag safety, a Marvel trigger, checkered wood grips, VQ rear sight, black post front sight on the Hunter and I drilled and tapped the mag release button and installed a 1911 mag release extension.