Reaching Lv40 is like reaching Lv22 on Mir2 lol.
Reaching 60 (Mir3) will be more or less the same speed as any other server having you reach Lv40 (Mir2) before content kicks in. Except with Mir3 in my opinion is able to keep you much more entertained with the amount of skills and stuff originally available before the new ones come into play.
If you haven't played Mir3 before, you have it best as it will be completely new to you with a load of skills you've never used before...you won't be waiting on TDB for the game to get interesting thats for sure lol.
Mir3 is much more balanced throughout than Mir2 ever was. The early game content is certainly not lacking, but if Jamie wanted to add more i wouldn't be against it by any means
But required to keep you busy up till 40-50+? Not really...
Then I personally haven't seen or played any of the KR content....then probably more on top if we get that far, pretty sure a lot of people will be stuck in for a while.