Whenever the President finds that, under the laws of any foreign country, citizens or corporations of the United States are being subjected to discriminatory or extraterritorial taxes, the President shall so proclaim and the rates of tax imposed by sections 1, 3, 11, 801, 831, 852, 871, and 881 shall, for the taxable year during which such proclamation is made and for each taxable year thereafter, be doubled in the case of each citizen and corporation of such foreign country; but the tax at such doubled rate shall be considered as imposed by such sections as the case may be. In no case shall this section operate to increase the taxes imposed by such sections (computed without regard to this section) to an amount in excess of 80 percent of the taxable income of the taxpayer (computed without regard