I think that after 1 season, provided he was able to spend a good pre-season with his Super Franchise so they could work on developing him physically into what they need, he would could be a pretty great player for Australian rugby.
Maybe, if he is motivated and willing to work very hard.
The question becomes, can you afford to pay him $700-850k on what would likely be a 3 year deal when you are probably only going to get the value out of the last 2 years?
The ARU probably cannot afford to pay a newcomer like Fifita those sorts of numbers. Apart from anything else, there are relativity issues with the whole of the squad. It would be interesting to know what every player earns, but we don't. However, my guess would be that only a handful earn over $700k.
So that would create actual and potential problems in itself. No matter if he becomes the greatest number 8 ever to walk the face of the earth, relativities have to be maintained. We need to put our best possible 23 onto the field, we cannot afford to lose established players because their noses are put out of joint.
If he wants to get paid very big bucks, he needs to stay in the NRL, if they will have him, or head to France.