I believe the Screamin Eagle will have good flow but also suspect it will be very loud. They are a fairly simple design with a simple baffle, no glass wool.
If your goal is to achieve a good sounding bike, you can forget about it. These big singles don't sound good. You will never get that V-Twin rumble. But if you want to make good power and improve fuel economy, then a free-flowing exhaust will help a lot. If you want the good power and improved fuel economy without making a bunch of obnoxious noise, then this muffler, or one with a similar design, will do the job.
This is a recording of the LCGP muffler in the garage. This particular muffler has a 1.5" baffle. There's a bit of chirp. The 1.65" baffle (1.65" ID x 1.75" OD) eliminates the chirp, but the sound quality and volume are about the same.
https://www.youtube.com/watch?v=m1e0M3u7i0M&feature=youtu.beThis is the same muffler on a country road cruisin through the gears. You can see that the engine revs free but doesn't make a lot of useless noise. You can make lots of power without offending all the other motorists, scaring wildlife, and being obnoxious.
https://www.youtube.com/watch?v=kkcUdPD0g_c&feature=youtu.beHere it is on the feeway. You can barely hear it over the road noise and wind.
https://www.youtube.com/watch?v=z7TGA-43JaA&feature=youtu.beYou don't have to use the LCGP muffler to make one of these. Any decent 2.5" glass pack will work as long as it has the same basic dimensions. If you make the baffle tube just like the one shown in this post, and perforate it with the same number of holes and the same size holes, it should perform just as good.
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