buying an alfa

Hi
I'm looking at buying an alfa and the shop that has them also has some
9dbi antennas going cheap but the guy selling them says that the
antenna doesn't give that much extra performance to the alfa.

Logic says that it should boost the power to equivalent of 4W (500mW *
2 * 2 * 2 by rough RF Maths).

Can anyone comment? I'm looking to buy tomorrow to get it for the
weekend so even any quick comments would be appreciated.

Thanks

Robin