Here's an interesting little piece I've just found in Google which nicely supports my figures.
Auto Exhaust Science
Written by some unknown has-been apparently but his figures seem to be correct.
You have to go right down to the Muffler Flow Basics section in which you'll find this.
"A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (1.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe."
I've corrected the typo where it actually says 10.5 inches of mercury instead of 1.5 inches. Now the deal about 1.5" of mercury is it's an old standard for testing carbs and as mercury has a specific gravity of 13.55 it equates to 20.3" of water. Now there's a number that's rings a recent bell.
Above I've already said that even perfect flow per sq inch at 20.3" would only be 124 CFM so 115 CFM per sq inch for a real piece of pipe of muffler length is bang on the money. 560 CFM being the maximum for a real piece of 2.5" pipe tells you without a shadow of a doubt that the advertised numbers of over 1000 CFM the OP referred to are blatant lies.
Dave