David,
As you may have read or heard in the past, it's all in the combination. I'm not sure, but I assume you have a fairly stock 5.0 engine.
Box style intakes like the Comp and Hartman generally improve power above 5000rpm over stock, long runner style intakes. Box uppers are designed for high RPM engine combinations. On a stock engine, with stock heads and cam, this intake would certainly make less power than the stock upper intake below that rpm. Unless you have the rest of your setup optimized for it, you will make less average power, and a little more peak power at the expense of much less torque and midrange. The engine would probably feel "peaky". The car would be comparitively slower in the quarter mile.
With better heads, more gear, and the right cam, this intake would work much better.
Check out the November issue of 5.0 Mustang and Super Fords magazine. There's an article comparing six high rpm intakes( two are box style ) at Anderson Ford Motorsports on an 8000rpm small block. The motor is a DSS 347 with an AFM N113, High Rev camshaft, and Stage V ported, Trick Flow, Twisted Wedge heads. Not exactly stock!
