The Porsche 960 will slot between the 911 and the 918's replacement. Probably in the $350-500k range and would compete in a space with the Ferrari 458, McLaren 12c and Lamborghini Huracan. Some might say it takes from some of the more extreme 911 (think GT2) sales, but Porsche sales are growing so fast across all their models I doubt it would even be noticeable.
Having this new "midrange" model will allow them to go even more extreme/expensive with the 918 replacement.