For decades, the 911 Cabriolet was the benchmark, but that’s because Porsche was trying harder than everyone else. Now they look like they’re tired of being the best, and Mercedes could be poised to take over, fake leather seats and all.

Before we talk about the AMG GT some more, let’s recap what the market is all about. At the top end, you have absurdly expensive cars like the SL 65 and Aston, Bentley Conti GT and Martin V…