For what it's worth, I shudder to place any value on this sort of speculative nonsense because it assumes to many things on an all-other-things-being-equal basis... It kind of assumes in this case that the Euro area cannot be reformed in a sensible way, when perhaps it can and is just a bit slow to get going. And they are assuming that Germany will never compete with Britain to acquire talented immigrants, which seems unlikely too.
But if we attract more migrants than Germany our economy will be fed with a labour force of young working age people while their labor force will shrink as the population ages. At the moment our GDP per capita is a couple of thousand US$ short of Germany's, and our population is smaller too. But in theory as we approach their numbers on one front, the other would follow. We would have a larger working age population which would push up our GDP per cap, and from the other direction theirs would be pushed down by the burden of more pensioners being funded by fewer workers.
More workers does in effect guarantee more GDP because of what that figure measures. But it doesn't measure as much as people sometimes think. Consider this, if you tore down all the million pound mansions in Kensington and built filthy hovels to replace them GDP would be boosted. This is because you would pay some guys to demolish stuff, you would sell some land for 1% of its original value, and you would then pay someone a pittance to build something cheap and horrible there. But every time any money moves, it adds to GDP, even though you have destroyed incalculable amounts of value in the process. So a terrorist with a dirty bomb could do a lot of good for the UK economy...
Making our economy bigger than Germany's is one thing, making it "better than" is a value judgement that I am less comfortable with than others are.
