[jonnyk]

I think a 2.5" exhaust setup is about right for a 5.0L up until around 500 horsepower. Then you would see benefits in 3".
Scavenging is just keeping the pipe small enough that exhaust velocity remains high. When the exhaust and intake valves are open at the same time, the higher exhaust velocity creates momentum and will vacuum in more intake charge, so you will make more power at lower RPM's with a smaller exhaust system. Granted, as RPM's climb, smaller tubing becomes a restriction as the velocity will increase due to the faster piston speeds. That's why high RPM drag motors don't run mufflers unless sound restrictions make them. They run just straight open headers...
You would have to get quite extreme with sizing for bigger exhaust to hurt PEAK horsepower...but it will definitely impact lower RPM ranges.
I personally believe longtubes and a 2.5" exhaust system should be the first thing you do on a stock 5.0L....the stock exhaust DEFINITELY restricts power output as has been proven time and time again with dyno tests.
Longtube headers will do better than shorties across the rev range as well.