Who told you they add that much hp? Do you have aftermarket heads and cam? If not, the max you'll get out of even a Performer RPM is 15-20 flywheel hp.
an aftermarket intake does make a very noticable difference on a stock car though, be it only 15-20hp. certainly no harm in adding one w/stock heads, especially if you do your own labor. then you can see the difference of stock motor, then w/intake, then w/everything else. its pretty cool seeing a car done in stages if you have to, of course if money permits slap a turbo on too w/everthing at once
Please don't take this the wrong way. By the looks of your thread count you are relatively new to the site or maybe Mustangs in general. When looking at aftermarket parts, keep in mind that some adds are very misleading. They are taking modded motors with a stock intake, then sticking their intake on it. Of course there is going to be a huge difference. So what does that mean to you, don't look at just the highest power potential. Have a goal for your car and then build it to meet the goal. A gt 40 intake is a great intake if you want to keep your car to a 302-306 with smaller aftermarket heads, and keep it somewhat simple. If you are planning a big horse 302- mild stroker, than a Performer may be what you are after. Look at the power the intakes make throughout the power curve. That will help you determine what you are after.