The thing is that even if we accept, say, 110mph as a "reasonable" top speed, the difference between that and a top speed of 140mph is not at all trivial, because it's not like the car just needs to be 30% more powerful, it needs to be in the order of twice as powerful.
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The real reason for these powerful cars is of course nothing to do with reasonableness, it's all to do with making them fun to drive for the driver, and that's exactly the thing that we should be never ever be optimising for, for anything that is allowed on the public road.