I've put slugs on all my mains after talking with several sailmakers and (non-J/30) racers. The concensus I got was the performance hit is nil, which makes sense when you consider the center of pressure is pretty far back. I made gates out of HDPE. I have not had a problem with hang-ups, but it stacks higher on the boom at the gooseneck (cover is tighter). Without slugs, I was constantly asking crew to re-flake the sail to avoid twisting battens. I missed one bad flake and ruined a few Ratbattens (switching this year to Bluestreak). I'd even argue easier flaking (slugs) results in better sail care and might have more uniform luff tension.
