I'm not inclined to real loud pipes myself, so my opinion may not carry much weight.
My 05 FXDL came with Screamin' Eagle slip ons, and they are fine with me. However, I was inspecting up inside them with a flashlight, and about 6" in from the back there is a metal disk blocking the pipe. It looks like they used a hole saw to perforate the inner pipe, except for a small section they left in place that acts like a hinge/mount, then just bent the circular section right straight up into the airflow. It your slip-ons are made similar, or you can scrounge up a pair someone is discarding, it gives you 2 low dollar options, use a steel rod to bend that flap back down, or a hole saw that is a tight fit in the inner pipe to saw it out. The direction of the bend and fold up operation they did is such that ramming a rod thru from the back end would fold that flap back down the right way. You could try it without even removing the pipes. There are probably other similar baffles behind it, but I'll bet on mine, I could drive a bar all the way thru, back to front.
First SugerGlide I had, (it was the very first SG, a '71) I did the same thing, but I used a rifle to soften up the baffles, an M1 carbine as I recall, about 10 shots end to end straight thru each slip on made the steel rod pass thru much easier.
Put Python I's on both wife's first bike, and my old softail. Loved them. Unfortunately they were bought by V & H and aren't the same. On her new Nightrain and my Dyna WG I installed Khromewerks slip-ons. Sound as good as the Pythons, and a hell of a lot cheaper.
I wanted a little more sound but not too much on my 07 FXDC. A friend of mine just put some Rush 2.25" baffle slip-ons on his Heritage and they sounded good. My dealer didn't have any of those for my bike but he turned me on to some older Screamin Eagle II's (NIB) he still had on the shelf and I'm very pleased with them. Made me a good price too to get them out of inventory I guess.
Personal preference is phor Vance & Hines. I've got a set of Big Shots Staggered on my 06, but I hear good things about the Bassini Pro Streets. Their a short-ish kinda pipe, and show good power on dyno runs.