The carb guys will hate me for this, but, if your carb is metering fuel reasonably correct for you engine you MIGHT gain .02 to .05 with a well built carb that flows the same as what you have. Most out of the box Holleys do a real good job when jetted correctly. Any gains would likely come from added flow capability. On a WOT bracket car it's not worth it. On a car with a throttle stop, you may gain some consistency with a built carb, you have to decide how well your current carb works. For carbs, there are few I would send them to, Davinci, BLP are the main two that come to mind.