How could it possibly NOT be the same on an iOS device - especially for a .0 release? I could see it not being the same on an OS X build since OS X releases don't typically correlate with hardware releases, but on iOS I don't see how that's possible.
They released the GM to the developers on September 9, ten days before the new iPhones will be in the hands of customers on September 19. The new iPhones would have needed to have iOS 8 installed on them at the factory, packaged for shipment, etc. long before the GM was released on September 9.
Apple may very well release 8.0.1 in the next 1-3 days alongside 8.0 to fix bugs that were found after the 8.0 GM build was given to the factory for installation (probably sometime in the mid to end of August), but I don't see how there is any way that the GM build released a week ago could NOT match what's installed on the phones or what's officially released to the public tomorrow.