Boris can make a fair argument because if it came down to every region keeping its taxes and paying for its costs, then London would be better off than everywhere else because the economic activity in the UK subsidises everywhere else. I remember seeing an infographic when I was at university which was a 3D map of Europe where the higher points showed where economic activity was highest, all European countries had a large spike up at their capital city but the UK's was frightening, it was basically a flat country, with a ridge in the south east going up to a huge pole in London.
However there is a key fault with Boris' argument in terms of 'urban giantism' which is a problem that blights a lot of developing countries where they have one huge city that dominates their country's economy - Santiago in Chile, Tehran in Iran etc, if you want to set up a business in these countries you have to be in the giant city because thats where the market is, which draws more and more businesses towards it, and hence more and more people come to follow, it becomes a giant sucking plughole that everybody is drawn into if they want to get a job or set up any kind of business, over time if you set up business elsewhere and rely on any sort of suppliers you can't get them locally because they are all in the capital. The result is you get a poor rest of the country and a capital city with enormous congestion problems, gridlocked roads, pollution and very poor standard of housing. London already has some of these problems and if you go down Boris' path it will end up like those cities.
