As with most urban locations, I suspect they'll be disappointed with the output.

I looked at a small (6kW) turbine as an option as part of a residential tower block refurb in central London, but it would have only provided enough power for the landlord's lighting if it performed as expected.

I think these things are often used purely as an indication of a building's sustainable nature, because most of the decent energy/carbon saving methods are pretty boring to look at, e.g. insulation, airtightness, double/triple glazing, biomass boilers . They simply say, "Look at me, I'm green!" ...when it would be far better to put that money together with the money from all the other small, badly performing wind turbines and build a decent sized turbine in a decent location.

Very good point