I've done a little chrono work with some 41 mags in 4 inch, 7.5 inch and 20 barrels. I'll have to dig it up. From my recall, I got a bit more speed out of the longer barrels but don't recall how much. Somewhat variable with the particular load. I agree with GB though, if you've ever read the piece "why ballisticians get gray" comparing a big batch of 357 mag handguns.
I also agree with GB on cutting the barrel, though I've considered it myself in the past. Better to trade for one in the length you want.