I think it would sell well in theory. But I wonder if BMW would be willing to put the John Cooper Works name they paid £££ for next to a diesel model? It comes down to badge snobbery really.
I personally would like to see a high powered diesel model in the range. The likes of VAG have been doing it for years, and SEAT especially have especially benefited sales wise from such models. Such as their 1.9 160bhp Ibiza Curpa which delivers 243 lb ft torque. It gets to 60 in 7.6 seconds, and the torque makes for very impressive in gear times, which in the real world count for more than the 0-60 sprint.
If they offered a Cooper S 'D' or even a diesel JCW then such an engine would be the perfect candidate. However as it belongs to the VAG group then I think its fair to say that will never happen.
And as with all VAG group diesels even the 1.9's / 2.0's still have plenty of scope for more after market power. I think AntFR on here will no doubt testify to that.
I am not familiar with the engine range BMW have access too, but from memory I don't think the Cooper D's engine has that much scope for
OEM power. By that I mean I don't think 160bhp ish from that engine could be offered from the factory due to emissions and/or reliability and I don't just mean the engine. The gearbox might not be up to the job either.
I'm willing to be proved wrong though.