How much will immigration help European fiscal problems?

Martin Feldstein thinks not so much:

The ageing of the population presents a major fiscal challenge for the countries of Europe.  The combination of increased longevity and a reduced birth rate will directly reduce the growth rates of the European economies by slowing the growth of the capital stock and by weakening the productivity of the labor force.  This slower growth of GDP means a smaller tax base and less tax revenue.  In addition, the current tax-financed systems of social pensions and health care will require substantial increases in the already high tax rates.  The analysis in this paper shows that the common prescription of increased immigration would do little to reduce the future fiscal burden.  The increased revenue from a large rise in immigration would finance only a small part of the coming rise in the cost of pension and health benefits.  The only alternative to significantly higher tax rates or substantially lower retirement income is to shift from a pure tax-financed system to a mixed system that supplements the tax financed benefits with benefits based on increased saving financial investment.

Here is the paper.  I can't find a non-gated version, which might be because Feldstein runs the NBER...