SB is correct in that factory manifolds, logs or RA won't provide much of a difference, in normal drivability. And if your eng. combo is relatively mild, with a stock drivetrain, and all you do is drive a few miles to a cruise nite, and show off some shinny paint, and you have a limited budget, then I would think twice about the investment.
"But", (and this is just me talkin out loud), if you've reworked your carb, and distributor for performance, have a step up cam, with some gearing in the back, and like to beat on it, (of course "legally", at a certified race track...
), then the RA manifolds will out perform the logs. Will it be noticable seat of the pants? Guess it depends on how old your pants are. But if you've ever put the two beside each other for a comparison, the visual difference alone is noticeable. And I'm sure the Pontiac engineers. when they designed them, wouldn't have cast them for their performance engs, if they didn't provide a benefit.