The 155mph top speed is actually a limiter fitted to the car. It's because of a gentleman's agreement between the big German carmakers – they've agreed not to sell cars capable of exceeding 155mph (although you can often pay more to remove this limiter). So, actually, your BMW 440i is probably capable of going faster than 155mph, and sitting at the limiter for a prolonged period shouldn't cause any damage. BMW tests its cars extensively at high speed so, provided you service the car regularly and keep on top of maintenance, there's no reason why driving at 155mph will lead to expensive repair bills. That's unless a lorry pulls out ahead of you, of course...