If you are getting combustion in the hot cat then that is from over fueling regardless of the cause. You are clearly getting enough air as performance is alright.

granted, injectors are checked at engine manufacture/install but one may have debris effecting the spray pattern from Morgan's install of the fuel system. This could effect the burn and if incomplete would account for the symptom of over fuelling regardless of the delivery volume. Further, I have seen ultra clean plugs from spray pattern errors. It might pay to find an aftermarket ford dealer who specialises in focus ST etc. They will be a lot better at finding this sort of thing out and you won't hear things said like "well it's not in the car it was made for is it!" Nor will you get some kid working on it!