After seeing this video of someone's stock exhaust modification and the beautiful sound it produced, I am seriously considering it for my Z4:
https://www.youtube.com/watch?v=lX8E0s--B8Q
The owner had the rear muffler box removed and the stock resonator replaced with a Magnaflow muffler.
Anyone here had this done? Someone in the comments said it cost roughly $600 for parts and labor, which seems pretty decent considering the cost of a full aftermarket exhaust system.