i don't know the numbers exactly cuz ive never had my car on the dyno. to give u a pretty good idea here is a link to an article i found:
2005 Ford Mustang V6 - Zex S197 Nitrous Kit - 5.0 Mustang & Super Fords Magazine
be sure and read all 3 pages. it shows the gainsfrom the nos kit with a 75 shot of nitrous. i have had no bad effects from the kit and i have only heard of people having something bad happen when they get greedy. just dont go more than 75 or maybe a 100 shot. ive added so many bolt ons here and there i couldnt guess what my horse power is. i know this though i am faster than the gt's. especially the 5.0's.