I'll bite on explaining this again.
Here is the deal. If you take a look at the size of the primary's on the 1 5/8" headers, what you'll find is they are MUCH larger than the exhuast ports on the stock head. What this does for you is simple. It "tunes" the exhaust flow. In other words, it actually helps suck exhuast right out of the engine due to the difference in pressure on each side of the connection between the head and header. When the headers tune the exhaust flow on a stock engine what happens is simple. It pulls air out that would normally be stuck for the compression cycle, and therefore, it reduces cylinder pressure. Cylinder pressure is one of the MAJOR factors in torque production. At higher rpms the headers can be more efficient due to the need for pulling waste gases out allowing for more peak air flow.
Unfortunately, the stock engine makes max hp at a rated 4200rpm, and max torque significantly below that. While the tests may indicate a gain of 10hp, the gain is recorded at 5500rpms, or past where you would normally shift a stock 5.0. Usually those test cars have some minor mods as well, allowing them to use the headers better, and sometimes major mods. Around town, a noticable drop in torque and low rpm drivability usually surfaces, the result of low cylinder pressure. When racing the meat of the powerband is very similar, usually with less hp and torque at and below peak hp production, and a little higher above peak hp. Adding a low restriction h pipe helps the header work more efficiently, but it's still a good mod, since that's where the real exhaust restriction is. Doing your entire exhaust system like you plan will probably help you a little, just because you'll gain a real 20hp from the H pipe. Without the H-pipe, slapping 1 5/8" headers on isn't gonna help at all. The header will drop your cylinder pressure, and the H pipe will slow and restrict the flow down the line. You'll wind up making less average hp in the meat of the powerband. The catback exhaust won't really do a whole lot on it's own either.
Making the headers equal length significantly increases the exhaust "scavenging" or "tuning" and will hurt low rpm torque significantly on a stock 5.0.
Long tubes are an exception to the rule. They are advanced in design. The thing that they offer that the shorties cannot is the ability to fully tune the exhaust. The longer and better developed collector creates backpressure on the cylinder that is on a compression stroke to hold the exhaust in the head, and on the exhaust stroke it does an excellent job of scavenging to allow for more max hp. What you wind up with is better torque, and better hp. A very worthwhile modification, especially down the road. The drawbacks are obvious, installation, expense, and access to the crap underneath the car.
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