Unfortunately, the only part of this post that is correct is the part where you admit you have no experience with the M54 engine. Headers are worth 10-20 hp at the wheels--depending on the particular headers and other engine mods--but modifying the rest of the exhaust does virtually nothing for performance here without removing the massive airflow restriction that is the OEM exhaust manifold. The collectors are very restrictive. They're designed to be quiet and lower emissions, nothing more.

I am unclear as to your point. You mention 10-20 hp improvement but then you qualify that with depending on the headers and the tune. There was no tune or well engineered headers in the equation. Exhaust manifolds are not designed to reduce sound or control emissions and certainly are part of the performance goals. Where does that come from?

I didn't say I was unfamiliar with the N54 engine. I said I did not know how the sound would change with headers but it wouldn't matter if I were not familiar with the N54.

My input is based on the universal benefits of an engine (a big air pump) from exhaust modifications. Stock exhaust systems downstream of exhaust manifolds restrict exhaust flow. That being the case any airflow benefit of headers will be lost without relieving the back pressure. Headers attached to a stock exhaust won't result in more air in because it can't get out.

Open headers would allow more out and an increase in HP though unless they are well engineered that can easily be a loss of TQ resulting in a longer time to go from A-B.

The higher the engine RPMs the more air is drawn in and pumped out. Open headers will allow more air out but unless the red line is increased the benefit is still minimal.

Since OP wants sound and performance but I don't get the impression he wants to get radical or spend thousands of dollars, my suggestion is to add a tune that increases the redline and improves HP and TQ. Modify the exhaust with low restriction mufflers, remove the resonators and enjoy. Time and dollars saved and more HP/TQ and better sound than just headers. However, I get the impression performance is secondary to sound. If that is the case I would just do the exhaust work and see if that met the goal. Since exhaust modifications can get loud quick and create a drone, might want to do exhaust mods in stages.

The M54s headers are cats, so aftermarket headers delete the cats. The rest of the exhaust system isn't all that restrictive.

Time to help out....
 

Hello my fellow e53 brothers. It's been a while. Having installed headers on my 3.0, however anecdotal, the old butt dyno felt quite a difference in performance and the sound is just shaweeeeet. The stock exhaust manifold with integrated cats is not optimal for performance. Bmw crammed the cats up high and close to the heat as possible to get them up to temp faster, to help make the m54 ULEV. I've posted picsl/vids in my other threads. They're quite compromised. I've also ran with open headers for a few days. There seems to be a loss of torque down low, but mid range and top end was awesome. Once I hooked the rest of the exhaust up, it was better down low and pulled much harder all the way to red line.

Just a note. I had shark injector software w/ the 7 k red line, modified exhaust with oem center muffler/resonator in place, and rear muffs replaced with some magna flow resnators welded in.

My X gets inspected this week, and then my headers are being installed soon on my current e53. I was going to do it over the summer, but I wanted some time to sort out aftermarket cats and Euro 2 software tune to delete out the post O2's.

^Glad you chimed in Ryan - yeah I remember you saying the power really got uncorked with the headers. Again if you don't need emissions testing then they really are a great way to go. If you do have emissions you'll need to fit high flow cats and play with o2 spacers on the rear sensor in order to avoid a CEL light (or using a tune like the Euro 2 which is a DME update) - note that aftermarket tunes that code out the rear o2's like Active Autowerke to the best of my knowledge will not pass emissions because it won't pass readiness

My experience
 

Had my headers on for about 3 months...
Power definitely can feel it and hear the engine now.
All threw redline it pulls hard.

And I have my setup only with k&n high flow with stock 3.0i back of the headers.

I am waiting to Shark it.
But how it is now it already increased the red line just with header install.
Before engine felt stressed if going around 4500 to 5k rpm.
Now I can recline and engine really puts the power down.

Strange thing for me is I'm getting precat o2 cel.
Bank 2 sensor 1 and I'm running the 02 90° spacers

Run the 90 degree spacer on the post o2 sensor only - the pre-cat o2 sensor should be installed right into the bung on the header.

This way the ECU sees the "difference" between the o2 sensors, there is a cleaner reading at the rear, the CEL should go away after a few drive cycles once you do that.

Sounds sweet! I just got my 4.8is exhaust on today. It sounds awesome:D. My headers are coming shortly. What type of aftermarket cats are you looking at? and where would you put them? delete the resonator?

sorry i was little unclear, i only have 90 degree on post. and the pre cat is straight in the header.
p0420 code.
i cleaned mass air flow sensor and o2 sensors are fairly new. maybe they are the bosch ones.
iv heard that bosch might not work with the spacers..

You should do a euro 2 tune :D Then you can delete the Secondary air pump and have no CEL.