As you have heard already a lot of people like the Z3M-type.
Maybe see that it at least Z3M fits (even non-M cars could fit M-exhaust for a small price...they are available on the spare market)
Some people may not want to spent the extra money of a new exhaust system.
Therefor, the exhaust that you supply should be considered an "upgrade" from Z3 and Z3M standard pipes.
Otherwise nobody will be willing to pay extra for it.
(you also have to look at the business case, right)