I stumbled upon the chassis code of the new 2015 WRX and STi while poking around my local dealer's website, and made an interesting discovery. The 2015 WRX and STi no longer share a related chassis code with the standard Impreza. But first, a little history:
Previously, the WRX and STi shared their chassis codes with the standard Impreza model, since they were really standard Imprezas underneath (chassis wise, anyway). This was the case with the GC and GD series.
GC WRX and STi
GC standard Impreza
GD WRX and STi
GD standard Impreza
Starting with the 3rd generation Impreza, the STi (in hatchback form) was graced with a different but related chassis code, GR. The standard Impreza hatchback received the chassis code GH. This was due to the STi moving to a widebody chassis, compared to the standard Impreza hatchback. The WRX hatchback joined the STi on the GR platform in 2011. The GV WRX and STi sedan arrived in 2011, with a different chassis code compared to the standard Impreza sedan (GE).
GR STi (GR WRX, GV WRX, and GV STi are similar)
GH standard Impreza (GE standard Impreza is similar)
Now, that brings us to the 4th generation Impreza, WRX, and STi. The new WRX and STi receive an entirely new chassis code, VA. As far as my research shows, this is the first Subaru to get a V chassis designation. The standard Impreza still carries a G series chassis code, which is GJ for the sedan. This a significant departure from previous generations.
My best guess is Subaru felt that enough significant changes were made to the body and chassis of the new WRX and STi to warrant a new chassis code. I would assume that the changes to the chassis were only as significant as the differences between the GR and GH, and the GV and the GE.
VA WRX STi (VA WRX is similar)
GJ standard Impreza
This would also explain the reports that were going around early on that the new WRX and STi were going to use a different chassis than the standard Impreza. This is actually the case, but it is still a modified Impreza chassis.