Y-Pipe = Power Loss?

I want to add a Y-Pipe exhaust on my V6, my car is stock, but my this guy I know said that if I add a Y-pipe, that I will have no back pressure and lose significant throttle power. How can this be if it gives you around an extra 10hp? Should I buy the new exhaust, or is he just full of it?

get a full exaust and maybe a cai that will be allot better for you

and if you cant afford it get an catback first at least just a y pipe wont do you much from all the backpressure you will still ahve in your exaust.