I was looking online for exhaust for my 2.7L regular cab. I have noticed that the Jardine exhaust is 2.25" inches in diameter as well as the collector on Doug Thorley's header. Since I cut off part of my exhaust few days ago and had the size in my mind, I had thought it is really close, if not exactly 2.25". So I went outside and measured the piece of exhaust I cut. I was NOT surprised when I measured exactly 2.25" in diameter.
There could be more power obtained from the 2.7L by having custom header made with larger collector and custom exhaust with larger piping made, however that system would loose low end torque and only gain horsepower on the top end around 4.5k rpm. Since this is a truck and it needs low end torque, system like that would be waste of money. I think that's the reason why aftermarket header & exhaust retain the stock piping diameter size.
My conclusion that the exhaust is waste of money since it doesn't change the diameter of the exhaust piping. The only thing which is different on the Jardine exhaust is the muffler. You are much better off, getting a muffler of choice welded in the exhaust system, which should run around $100.
Now where I think the real gains can be made is getting rid of the two catalytic converters which are located on the pipe which is located before the exhaust. Have those cats cut off and replaced with straight pipe. To get rid of the CEL light, space the 2nd o2 sensor with two spark plug anti foulers, which can be found at local autozone. **DISCLAIMER** Cutting out the cats will make your truck fail emissions, if your emission station does sniffer test (hooking up pipe to your exhaust). Here in AZ, they just hook up to your OBD-II port and if your vehicle doesn't have CEL, you will pass. My previous car passed emissions here with no catalytic converters.
This all can be done for less than $200, if you can weld maybe even less than $100 and it should yield the same amount of horsepower as getting a Header for $450 & exhaust for $280 new.
Looking on the stock exhaust manifold, I don't think replacing it with aftermarket header will yield much horsepower anyways, maybe 3-4whp, since the stock exhaust manifold doesn't have a catalytic converter like the 4.0L does. The Doug Thorley header has 2.25" collector, which I believe is the same as on the stock exhaust manifold... The only difference is the design, where the Thorley's header has longer runners before the collector and that is the only benefit of the header.