I've gotten up to ~5500 before, but I'm not even level 30 on the isotropish leaderboard.
It's swingier, but that's probably the main difference.
There's some sort of a bias as well, though I don't quite know where it comes from.
I'm constantly around 28-30 on the isotropish leaderboard, yet around 4500-4800 in Goko, roughly never reaching 5000. Compare that with sudgy's figures and you see a clear difference. What makes this notable is that I have mostly played bots (since I can take breaks when playing against them), which hints that the Trueskill-based ranking treats someone constantly playing against average players (the bots are between level 10-15) more favorably than the Goko ranking does.
The Goko ratings severely penalize losing, and only mildly reward winning. To stay above level 5000 you have to either be really good, or really careful about who you play. I should be about level 28-29 (I got up to 30 on isotropic but I doubt I'll do it on isotropish, the amount of players available for a game around my level is too low)
I got up to 5500 just by consistently beating level ~4500's and getting lucky once with a 6000+. I went down to 4500 by losing to several 6000's in a row... it's confusing.
You can't even play the bots if you want to stay above 5000-- it doesn't really do anything if you win and you lose a lot if you lose..