IMO, the Epik would be an upgrade for sure.
I think that since the Mk3 is already a 12", all you are going to gain may be some output compared to the two in question. If the VTF-2 is struggling, then yes, they would probably be upgrades, but I'd probalby be looking to step up a bit more so you don't spend $800 for a marginal upgrade. Have you considered just adding another VTF-2?