i have a vectra 1.8 16v thats LPG, i found a big difference in power when on gas, it is a factory fitted install and runs beautifully.
i dont think that you will notice too much with the engine in your bmw but i did on my little 1.8.
sometimes on motorways i flicked the switch back to petrol for those hill climbs and found it towed better on petrol.
i now use it as a spare and went diesel for the main tow car, but dont let me put you off, lpg is one of the best things you can do to a car in my opinion, even more so in a big engine like yours. i am sure it will be perfect for you.