My son-in-law has a thirty round mag for his Glock. Sure defeats the purpose of having a compact size/light weight pistol. I guess it would be nice if you are in a fire fight. I used to laugh at my fellow officers at their concern for having enough reloads when they were carrying a Barretta with 16 round mags, and all I had was a six shot revolver. Even though I was just messing with them, I liked to tell them that if they knew how to shoot straight, they wouldn't need that many rounds.
I do know that there is a company out there right now that makes plastic magazine extensions that add one or two more rounds. I haven't found one made for sigmas yet. Everything seems to be made for the GLOCK. Darn evil, foreign gun!!
I have seen folks on youtube that have spliced two 45cal mags together successfully. I guess anything is possible.
I understand that there are a couple of states that restrict the capacity of magazines. Before modifying your mags, you might want to check your state.
Good luck, and please let us know how it works out for you.