Those would be an option for sure. In order to get serrations you would still have to send the off and it gets expensive. Here is a link for a place that does serrations.

Serrations - S&W M&P - Innovative Gunfighter Solutions

My thinking is that you could sell an aftermarket equivalent slide for less than the price of the defensive package. If someone was set up to make slides they could also make longer and other variation slides for competitive shooters and others who want them.

Looking like I am wrong however and Glock may be the better option for those customers.