So where does the stock placement of the MAF on the 5L fit into this equation? There aint no tube before it - its just airbox!

Hence the reason why they offered things like alloy bellmouths into the aircleaner box to try and smooth the transition. The Pro--M's are calibrated on the airbox so they should be far more accurate than the Ford ones... The standard airflow meter is a close enough thing - it is actually a mustang part rehashed to fit into our engine bay (like the rest of the 5.0 install). That is why most Falcons run a little on the rich side of stioch. Close enough with air fuel ratios in a standard car is fine - but when you start thinking about a lean condition in a boosted application and the damage incurred - how "close enough" do you want to be?