If you haven't got HK, and you really can't live without a fantastic hifi, go to a specialist car HiFi centre and get the existing 'system' enhanced.
There are many high end speakers that can be fitted where yours are now and under seats etc along with an amp could make a hugh difference.
If you spend a lot of money to upgrade to the Alpine or whatever, and it does not meet your expectations then what. Do you trade-in your 1er on a another 1er with HK, Zenons and all those extras in that colour you really liked.
Live with it for a while longer, you may just get to like the one you have.