Ok, to bring this thread up to date (took my time, lol)....
I took the plunge and bought two UE BOOM 2 speakers for various uses, including in the mini.
I am very impressed to say the least!!!
The speakers are more than loud enough, and are small enough to put wherever I like. The sound is epic!!!
AND, you can link up to 50 speakers together, wirelessly, and play your music through all of them simultaneously! (Though 2 are more than enough).
If you only use two speakers, you can set them as a stereo pair, so that you get proper stereo sound (not just mono).
They also have a 12hr battery life. More than enough for a day's driving about.
I can say that I am converted. I will never be doing any kind of stereo install on my mini. There is just no point.
To add to the happiness, you can take your speakers out of the car and bring them down onto the beach, etc, etc, and continue listening to your tunes! Win win all round if you ask me. They are massively rugged, and fully waterproof (they can be dropped into water). They have bluetooth connectivity or a jack connection. They have handsfree calling, if you need it. They also have integrated mounting nuts for if you want to mount them on a camera tripod, which means that I could probably just install see M6 bolts somewhere in the car, so that I could simply mount them (possibly on the rear parcel shelf??). What's not to like?
(You'd think I was a Boom 2 salesman, haha!)
Thanks for all your input, people.
Good luck!