I prefer the Dynomax Super Turbo mufflers.They flow extremely well foor a reverse flow type muffler.I think they give cfm ratings for them on their website.I have a set of the 3" large cores on my 440 equipped dually and love them.
I don't personally care for the Ultra flows,they are too similar to a glasspack in design and sound for me.
I used to like Flowmasters,but the older I get the less I like the noise in something I put any real mileage on.
Keith