Space is at a premium in a Miata. Trying to find a place to mount seven speakers that will do justice to the Bose system will be like trying to park a Cadillac Coupe Deville in a motorcycle parking stall.
Even if you can get that feat accomplished, how much room will be left in the trunk. I am not sure that you could put a Mc Donald's Kiddie meal in there, let alone a spare tire.
Now I own a Miata and to tell you the truth, the road noise is preferred by me to the stereo. Just put the top down and go for a ride on a country road, the surround sound system Mother nature gave us beats any recorded music that I have ever heard.