I always thought that coupé, which is after all the French word for "cut", is one of those things that can't be defined by itself but only in relation to other things. So a Vauxhall Calibra is a coupé version of the Cavalier, because they took the Cavalier and cut some off to make it more aerodynamic. Rover 800 coupés were cut-off Rover 800 saloons. That BMW, because it has just the same roofline as the 4-door, would still be a saloon by that reckoning. According to Wikipedia, at least one car (the Mark 1 Ford Granada) was available in both 2-door saloon and 2-door coupé versions, both appearing from the photos to have four seats -- which would argue against defining coupéité in terms of door or seat count.

Peter