THIS IS NOT your classic winner-takes-all comparison. When you pit 225kW against 310kW, performance cannot be the sole decider, or this story would end right here. What we’re trying to find out instead is how the most potent compact hatchbacks available from BMW and Mercedes fare when you pull out all the stops. To do so, numbers took second priority subjective impressions. The stopwatch was replaced by the body clock, the shameful filling station receipts were binned with an impish grin, and our driving licences were left soaking overnight in holy water. After a longish wait, it’s at last crunch time for the Mercedes-AMG A45 S on home turf, with a blue BMW M135i looming large in the rear-view mirror.