Well, technically it will be an improvement;

Siege of Orgrimmar lasted 429 Days, ICC/RS lasted 364 days.

If Legion launches as early into Summer as possible (which I think is likely since we are already getting Beta) a June release date would mean Hellfire Citadel will last for 363 days.

So technically Legion would be arriving over 2 months earlier than the MoP->WoD transition which is undoubtedly an improvement.