Underdrive pulleys

[mpk490p]

Hi all,
i need to get a double belt alternator pulley and didnt know if it would be worth getting a underdrive set with main and alternator, i dont use the car everyday but when i do and its a meet i can imagine sitting in a lot of traffic so i was wondering is the bhp gain worth the risk? is there really much to gain from using them?
http://www.mazdatrix.com/b9.htm
thanks
Mark