This is another one of those things that makes squat's worth of difference.
If you had a drilled slide and I took it out without telling you I'd bet serious money that you couldn't tell the difference.
Some people claim that if you drill the hole just right it works good, if not right the slide will bounce badly and make the bike run funky at certain rpms.
But....if you get into this part of the carb there are a couple things to watch for:
1. If you drill the hole make sure you
clean up any burs that the drill bit leaves behind. A bur in the right place on that hole can seriously screw with the action of the slide.
2. When you put the white spider looking spring seat thingy(sorry,it's early, can't think what they call it??) back in the slide, make sure none of the legs is covering the hole that you just drilled. This will also screw with the slide action.