Sounds very strange - on the German website 0-100 km/h (0-62 mph) is 5.4 sec. for both variants. Only performance difference indicated is 2 km/h higher topspeed (274 km/h) for the RS60.
Though the accuracy of the shown numbers sometimes is doubtful indeed: weight for the RS60 is indicated as 1,355 kg which is exactly the same as the standard Boxster S. Can't be right, as the RS60 has 19", PASM, full leather etc. on board whereas the weight for the stock 987S should include none of these options