Ya.. You cant just toss a cam in a stock ls3 and get 50 extra HP for a $300.
You need to do ALL the supporting mods first. And don't forget the custom tune thats about $500.
O ya.. and I would go with thunderracing. I loved their 224/224/114 cam in my LS1. with all the supporting mods, I got 395RWHP. and they have the best customer service too.