Results will be coming very very soon. And as for whats better, who knows. Can't say until it gets on the dyno and even then it will be best guess. But 800 bucks or whatever they cost for the Kooks, or the $250 I have in mine, thats where I win. And even if you bought a new set of headers for 400 bucks or whatever they are and modified them your still ahead of the game. So say all the gains are equal, cheaper wins. Unless you have no skills and/or no desire to do the modifications and won't do anything except bolt on, then Kooks are where you need to be. And not saying there is anything wrong with that, no headaches or effort is fine, I usually prefer that road, but I refuse to spend money foolishly when I know I can make something just as good if not better for less than half. Just my viewpoint though.
These are the prices new for the Kooks and the Mac Long Tubes...
Kooks 3/4 TBird headers:
$770.00 - SS 2.5"
$845.00 - SS 3"
$826.00 - Jethot Coated Steel 2.5"
$901.00 - Jethot Coated Steel 3"
$500.00 - Jethot Coated
$400.00 - Chrome plated
Even if you buy used, the MAC's can be had for $100.00, I seriously doubt you will find used Kooks for under $350.00. I am not arguing that the MAC are cheaper, they are. My point is this.
The MAC Long Tube headers are NOT PROVEN to be better or even comparable to the performance gained with the KOOKS headers which is known. They MIGHT perform as well, they MIGHT perform better or they MIGHT perform worse. Until the gains/losses is shown with the Mac, NOBODY should be claiming they are better or even the best option.
Spouting off again? Anybody who knows anything about exhaust or has USED exhaust systems and proven how they perform will show you the benefit of long tubes. Why do you think the Kooks mid-lengths are better than the shorties? Do you have any idea what is actually going out? Do you think they "flow" better? Because that's not the answer. Do you think there is less "back pressure", well that may be true, but that ain't why long tubes are better than mid-lengths or mid lengths are better than shorties, either.
You're right, I haven't directly Kooks to long-tubes on a 4.6L engine. But I HAVE compared long-tubes to ANY exhaust on misc Chevys, Fords, & Dodges. The trick has to due with with the Equal length tubes and resonance frequencies. An engine pounds out sound waves through the exhaust - it's just the nature of the beast. With equal length long-tube headers, the reverberation (or the waves comeing back to the engine) are "in-tune", for lack of a better term, and assist scanvaging of the cylinders and flow. There are many more better descriptions on the Internet of what is going on, but I'm not that technically wordy to describe it perfectly, but I'm sure you can do a google and read all about it.
What I can tell you is this - EVERY SINGLE VEHICLE I've ever been in that has had long-tubes added (no matter what previous exhaust system was installed) has a seat-of-the-pants improvement and turn better time slips. So, I have seen and felt the results and I stand by my experiences and beliefs.
What happens with older cars is not nessary true today. Take a good look at Kris Danner's 2011 Mustang GT. His car has the factory headers and factory airbox, but a modified exhaust. His car is making the same power as cars with aftermarket CAI's and Long tube header equipped 2011 Mustang GTs. He has one of the fastest cars. Guys are running the stock headers on 500+ N/A HP 5.0 2011 Mustang GTs. Long Tubes do not seem to have any benefit on the new 5.0L.
Step back a generation. The best performing headers for the 4.6 3V is a Mid length and a premium priced Long Tube. That debate is heavily contested on both sides as to which one is better. At the end of the day, it is so close that it the best is which ever one you feel like having.
On a 1970 Ford Boss 429 Mustang, I would agree with you that Long tubes would show a significant benefit/gain. On a today's muscle car, not so much. I think for a Long tube to show a benefit on a today's car, there is going to have to be some other serious mods to the car before it will show any benefit.
Again, I will refer to the 3V mustang world. For the Long Tubes to show benefit's in the 3V mustang, you are looking a full out N/A build (400 HP+) or a heavily modified SC'd car. If you don't have either of those, the Long Tubes are of no benefit to you.
On a bone stock 4.6L TBird, I am willing to bet that adding Kooks to it will have little or no benfit to it. For the Kooks to show a benefit, there needs to be some substantial mods to the car.