Installation of Steeda underdrive pulley affect supercharger boost?

You could always swap out to a smaller blower pulley to bring boost back up to previous levels. You'd have to check to make sure you're not running too small a pulley though, otherwise you'll suffer from a loss of traction on the belt because of the smaller contact surface area of the pulley.

If the stock 3V crank pulley is the same diameter as the 2V, then it measures about 6.85" (which it may, or may not be. You’ll have to measure for yourself to know for certain)

D = Diameter of the Crank pulley
BP = Diameter of the blower pulley
R = Over all pulley ratio

6.85 (D) / 3.2 (BP) = 2.14:1 (R) pulley ratio.

What you want to do is measure the diameter of the under drive pulley, then swap out to a smaller supercharger pulley that still gives you the same ratio.

ex...if the under drive pulley is roughly 5.0" in diameter (which is just an example), then you'll need to do a little reverse math to calculate the size of the new blower pulley to give you that same ratio. 5.0 (D) / 2.14 (R) = a 2.336 (BP)

My guess though, is that you're going to see some belt slip with that small a pulley, unless you go with an 8-rib conversion....which isn't going to be cheap.