That's not Amdahl's law in reverse, that's just Amdahl's law as applied to mulitcore. This was all worked out over a decade ago [1].
[1] M. Hill and M. Marty. Amdahl’s law in the multicore era. Computer, 1998.
[1] M. Hill and M. Marty. Amdahl’s law in the multicore era. Computer, 1998.
http://minds.wisconsin.edu/bitstream/handle/1793/60554/TR159...
And second that was done in 2007 (did they do an earlier version?)
Its worth a read.
http://research.cs.wisc.edu/multifacet/papers/ieeecomputer08...