To find out, I took the Glock 35 I had converted to 9mm, and converted it back to its .40 configuration, and then dropped in a 5.32" Lone Wolf .357SIG conversion barrel.
I was dreading doing the re-converting, because I had even changed the extractor and trigger housing (which contains the ejector) to the appropriate 9mm parts, but I needn't have worried. Even my ten-thumbed self had the frame and slide stripped down, the parts swapped, and the gun reassembled in fifteen minutes.
American Eagle 125gr FMJ
Federal HST Tactical 125gr JHP
Hornady XTP 147gr JHPThose averages are, respectively, 53fps, 36fps, and 55fps more than the shorter-barreled gun. It's not a ton of improvement, but it's there. The long-slide gun also shot a whole bunch flatter than the compact 32. The small gun borders on being a handful, while the bigger one is almost docile. Without having yet tried them on the clock, I'm betting splits on harder targets, such as steel plates at 20+ yards, will be noticeably quicker.