Sorry it took me a couple times reading this, but now I understand your point of view, so I think I can equate my terminology.
I associated "OEM mod" with any form of cutting open the stock unit and messing around inside. So I thought of it as a single "OEM mod" with two different means to the same end.
I understand "straight pipes" to mean simply that. A straight (or properly bent, in our case, to go around the differential) piece of pipe with absolutely no purpose other than to provide safe pathway for the exhaust gases past the rear end of the car. Similar to test pipes that completely eliminate emissions controls.
Either way it's done ("OEM mod" or "ACM mod") the Helmholtz chambers are left unmolested and the stock muffler unit is returned to the car to maintain the appearance as if no tampering ever occurred.
If you are asking about completely removing the entire stock axle back unit where it connects to the mid-pipe and replacing it with straight pipes only, then I'm fairly certain the sound won't be exactly the same as the OEM/ACM mods and you will have more drone.
The key is in the Helmholtz chambers. http://en.wikipedia.org/wiki/Helmholtz_resonance
Their job is to generate specific frequencies that mold our cars' unique sound while also cancelling out drone.