The AP gain is exponential, if you're behind on artifact knowledge then you'll get an AK every 3 days which means getting power at 10x the normal rate after 1 month, 100x the normal rate after 2 months, 1000x the normal rate after 3 months. Right now this caps at 250x, but that cap is arbitrary.Legioners will be ahead a coulpe million points compared to newcomers.
We'll get a lot more than a few million AP in Legion. IIRC it takes 5.2 million to max an artifact and 70 million to max the prestige level.. 70 million at max artifact knowledge is equivelant to 280k with no artifact knowledge.