Have you got a source for this claim of low 5sec? It has no correlation with any test I have ever read for the RX8 - save for an article on a custom TURBO RX8 in one of the Austrlaian magazines.
Realistically it is 14.5 to 14.8 over the 400m and 6.5 to 6.8 to 100kph.
Why do you think all the RX8 owners were complaining when the MPS3 was released? It is because the MPS3 is faster in a straight line. 5 sec is bull.