LS1 underdrive pulley question <> and other mods?

I have a stock 2000 ws6 and have a under drive pulley that i have yet to install but heres the questions. Is it hard to install? How much should i expect to pay to have it installed? Will it slow my accesories down to much? ANd the big question is how much HP will it gain? Ive also got a smooth air bellow and whisper lid, throttle BP and CAD to install. Will I see much from these mods? Thanks in advance! kirk