If the rating system is an ELO-type rating system, which I believe someone from Goko said it was in a FAQ, then there's no true way to match ratings on goko to levels on isotropic.
The more active players there are, the higher the top ratings will be. More overall rating points will be in the system, allowing the scores to become slightly more spread out. As more and more people create accounts and participate in the rating system, the points will slowly gravitate towards the top players, increasing the top possible ratings ever so slightly.
The ratings that can be reasonably achieved, therefore, are determined mostly by the number of players. The only way someone would ever be able to reach 9000+ is if many, many more players joined in the rating system. If the number of players remains steady, it will be difficult for any player to get above the highest current ratings.