Heat flashing is when your exhaust causes the coolant in your EGR cooler to boil and pukes out your degas (coolant reservoir) bottle. It's fairly common on tuned 6.0L's. The block off plate should eliminate 90% of the problems with the EGR cooler and since you don't have the scoop in your up-pipe, you only need a block off plate or a freeze plug.
Advantages-no coolant puking; won't have to worry about blowing up your EGR cooler and won't have to worry about leaking coolant into your heads possibly hydro locking your engine
Disadvantages - None that I can think of, cost and/or labor to install it maybe.