I would say that 10 to 15 additional horsepower would be more realistic. A supercharger can add a 25% increase in power but I doubt that the smaller pulley alone would do that. The charger on the X increases the 3.3's output by only 15% on the top end and 18% on the low end.
The down side to the smaller pulley is that your supercharger and it's bearing have a lifespan. The faster you turn the charger the faster it is aging. Enjoy the additional power for a season but expect repairs down the road. I tried the smaller pulley thing on my Mustang and met with repairs that sucked.