Forget 28hp from a chip on a NA car, it's not possible and if it is it'd only be on cars with race engines.A few of us here run GTI's with chips from pepeoi, a seller off ebay. Mine was dynoed with and without a chip and there was a 9hp difference.
Yes. It's not just 9hp, but more torque, more linear power and also better mpg when driving sensibly.