While a bigger engine will always make more power, the same size engine with twin turbos and DI willmake more. More valves is typically better, because out 5.4 motors are 3 valve engines. Ford even made a video about why a 3 valve is better than GMs 2 valve, so I will let them shoot thier feet and explain it to you themelves
But you don't need multi valves with this much bore space. Multi valve have a larger advantage in small bores due to shrouding issues that you would run into with larger 2v. 400 plus cfm has all ready been accomplished by working the 2v 6.2l ports over. Who the hell needs even close to more than that on the street?? The ecoboost will probably make more power per its size. But who gives a shit? This isn't a Honda forum.
Of course Ford made a video trying to convince why three is better than two, their trying to sling a product.
Yes, multi valve heads will flow and be more efficient 99% of the time. But due to the 6.2 large bores, there is no need for it. Unless your trying to make 2500hp, which no one here is doing.