to know that, you have to know how accurate your current MAF is and it's response rate. The response rate of the new style MAF's are far better than the stocker on it's best day. As far as the calibration and accuricy of the stock unit, that's case by case depending on the deviation. The HP gain will be solely dependent on that.
If you have to have a 'holy shit!' moment on the butt dyno to justify upgrading, don't bother. It's not that kind of upgrade (until the stock unit dies)