'E+' is not an official designation. Actual aircraft would be designated F-15E, but this possible variant represents those that are equipped with a pair of either the more powerful Pratt & Whitney F100-229 or General Electric F110-229 engines. These engines have been installed in all F-15C's produced since 1989, so it's highly likely that they will be installed in production F-15E's soon. [See also F-15E]
