The E46 manual and brochure seem to suggest that the engine will produce maximum power only if 98 octane petrol is used, and that the power will correspondingly be adjusted downwards the lower the octane.
The above could perhaps be tested out if the car is filled up with pure 98 octane petrol in Singapore. In Singapore, they don't have 97 octane petrol like in Malaysia - theirs is 98 octane. I think they even sell 100 octane petrol there, if I'm not mistaken, which some JDM cars and direct-injection lean-burn cars require.