He explained that even in a land and groove barrel, unless you use FMJs or hard cast bullets, the velocities of 9mm Luger bullets is such that a lead bullet would shed material considerably in any barrel, polygonal or not, resulting in possible accidents.
~1,100fps is not really very fast as lead bullets go. If you use quality lead bullets that are properly sized and lubricated, not too soft, and loaded properly, you should not have a problem in a conventional barrel unless you do something inadvisable like shooting multiple thousands of rounds between barrel cleanings.
I've shot a couple hundred rounds of cast lead through a 9mm pistol in a single range session. There was some
visible buildup, but it was pretty minimal, and it was easily cleaned out with regular Hoppe's #9 solvent and a newish bronze brush. I didn't even have to get out the Chore Boy pads or the lead remover.
I've been told that polygonal rifling demands more frequent cleaning than conventional rifling if cast lead is used, but I don't have any personal experience with it, so YMMV.
[EDIT TO ADD NOTE: Yes, as with the other posters, I'm also talking about hard cast. Unalloyed "dead soft" lead will
cause buildup at 9mm velocities, but this is common knowledge among handloaders, and most reputable bullet companies don't even offer bullets sized for 9mm unless they're hard cast.]