Assuming you're asking about CV carbs- changing the open area in the slide lift holes affects the rate at which the diaphragm sees changes in engine vacuum.
Drilling the holes out larger makes the slides react faster, while plugging one of them (as is done with some DynoJet kits) will slow the rate of change.
Drilling the slide holes might (emphasis might) sharpen throttle response in some applications but it's also just as likely to introduce a midrange bog... at which point you're going to have a bitch of a time returning them to the stock diameter.
As long as the slides were opening fully beforehand there is no way for this mod to add horsepower... in short- probably not worth the bother.
Nothing you do to the slides can add peak HP... unless, of course, they were fucked-up-as-a-football-bat to begin with. The slides cost about $35 ea. from Ron Ayers, and they are some sorta phenolic plastic that seems to be somewhat fragile. If you're not having throttle-response problems, and your midrange is nice and smooth, leave your slides alone, they're doing what they're supposed to do.