the following information is based on the investigation into this matter, that appeared in the German section of EU forums, specifically the data by Schrottkiste and General_Jack_D_Ripper (thanks for letting me know, General :) )
The outcome of the abovelinked thread is:
Premium tank XP bonus modifier (that was introduced not so long ago) does NOT count towards the crew XP, used in that tank.
That also means the “graphs”, used in the abovelinked WG post are useless, because they do not represent the real training curve. General_Jack_D_Ripper explains:
Before battle crew XP status (right-click and “view image” for full size):
After battle crew XP status (right-click and “view image” for full size):
Crew XP actually gained: 2820XP (or twice for “Accelerate Crew Training”).
Tank XP gained: 940XP * 2(daily) + 657XP for using premium vehicle) = 2537XP -> Crew received 940XP * 2(daily) *1,5(crew-XP modifier for premium vehicle) = 2820XP
Crew should actually have received: [940XP * 2(daily) + 940XP * 0,7(Tier II premium vehicle XP modifier)] * 1,5(Premium vehicle crew XP modifier) = 3807XP
The original WG post advertises, that players need * 2,55 less battles(1,5 * 1,7) in Tier II, while in reality it is 1,5 less battles for ever Tier. As a “bonus”, if a player uses Accelerated Crew Training, 100% of XP “for use of a premium vehicle” is lost.
This bug was confirmed by WG support staff (Mr. Luke Vos) and passed on to WG developers for fixing.