Skipness One Echo:
Absolutely agree with you, the runway extension will not pay back the investment in the short, or even the medium term, but equally it probably will pay back more quickly than it will do at Cardiff -which I seriously doubt will ever turn a profit without some pretty creative accounting (that is the runway extension, not the airport itself).
San Francisco is a complete no-no - you would have to assume that any destination westwards that MAN cannot make work, won't work from BHX either, so that leaves the least unlikely destinations as being New York, Chicago, Atlanta and Orlando - of those I can only see Chicago and Orlando as realistic possibles.
Looking East, and if Manchester cannot sustain direct services to Singapore and Hong Kong, how is BHX supposed to sustain Beijing or Shanghai?
I'm sure that on the ITX front, there will be benefits with direct services to Cuba, and more choice to the other Carribbean islands, and also to destinations like Goa. These alone won't pay for the runway!
What the runway will give is the chance for the airport sales team to market itself effectively to long haul carriers, which it could never do without it. How effective it (the sales team) will be remains to be seen.