Underdrive pullies are a bit of a sore subject on these Engines. I never had a problem but some belive that removing the standard harmonic crank pully can cause the crank to flex, thus shattering the oil pump gears and killing the whole engine. I used to drive my ST170 how they were meant to be driven and with an oil and filter change every 3000miles never ever had a problem. Most reliable car i owned.
These wont give you more power but will add to the effect of a lightened fly wheel by removing rotational mass from the engine. They will and do allow the engine speed to increase quicker, but its only noticable in 1st 2nd, the gains from a lightened fly and pullies will not be noticed past 3rd gear. It certainly helps your 0-60, i could pull sub 7 second 0-60 times all day long.
I had a CFM short shifter which was absolutely awesome too, made the gear change so quick and short, helping 0-60 times of course and if you remove the counter weight from the linkage too it makes for a better gear change.