I'm not up to speed on all of the companies that are offering 128 or 135 parts, but when it comes to an exhaust, a lot of the parts are universal. To me the most crucial part of the exhaust is the diameter of the pipes and the type of bends or even the lack of them.
The muffler can be tricky just because there are so many different types of internal design - straight through, chambered, or a combination of the two.
A lot of the straight through mufflers have perforated tubes surrounded by fiberfill, while others, like Corsa, have frequency trapping chambers like a gun silencer, but is otherwise straight through. The options could be endless.
Here is a
company that may have something for you. They don't list any BMW systems, but they do sell mufflers and nice double walled tips. AWE Tuning uses them for their VW systems.
I know you said you want loud, but you don't want a resonator. The resonator is there to "shape" the tone and eliminate or reduce unwanted high freqs. Not using a resonator can open the door for harsh or buzzy tones. Most resonators are straight through flow with a perforated pipe surrounded by an empty can.
Who is contracted to make the BMW performance system?
Also, why not use a 2.5" IN/OUT. 2.25" seems a bit conservative, especially for "loud".