I believe what is being missed by most here is that these misfires are occurring on only one cylinder. The PROcede does not have ignition or fuel control per-cylinder; it adjust the input signals which the DME alters the timing and fuel for all cylinders. So, a wiring issue or boost target (different map) is not likely to be an issue. This sounds like a pure mechanical function which may be difficult to resolve.
Since the plug was replaced and the coil packs moved and the problem persisted then the other area to look at it the fuel for that cylinder. Not having pulled these DI's I do not know how difficult they are to pull but that is probably the next step. Move an injector and see if the problem moves with it.
If it stays with cylinder 6 the problem could be more significant. An improperly seated valve, etc. This is most likely becoming visible under the increased load but the problem has always been there. Heck, even a poor casting of this head could cause the issue and under stock loads, it may never be seen or may take longer to become visible. But I would hope it is just the injector at this point.