Here, from the most recent work done on Morelia relationships:
"So, in conclusion, our data are minimally consistent with monophyly of Morelia. We find three lineages within Morelia: M. boeleni, the M. carinata + M. viridis clade, and an M. amethistina + M. bredli + M. oenpelliensis + M. spilota clade, which concurs with Kluge only in the sister relationship between M. carinata and M. viridis and the somewhat remote position of M. boeleni."
Rawlings et al (2008). Python phylogenetics: inference from morphology and mitochondrial DNABiological Journal of the Linnean Society, 2008, 93, 603–619.
This means 1. nothing about carpet pythons as a group, other than they are part of a big mob of Morelia pythons, but very different to M. bredli, M. amethystina and M. oenpelliensis, the only other Morelia species. 2. M. viridis (South), M. viridis (North) and M. carinata are in a different genus altogether (Chondropython) and that M. boeleni is it's own thing.
Anyway, back to the point, the fastest growing carpet python, do you mean by mass, length or rate according to maximal size? Basically the bigger carpet pythons grow fastest, such as M. mcdowelli and M. spilota, and of course their intergrade zone representatives! Although the fastest grow should occur in M. s. variegata, as it has a higher metabolism, but it will slow down in growth sooner than the other big species due to its tropical biometrics.