In my opinion - only if the cap doesn't get raised periodically.
I think a level cap to 55 for the first 3 months or so wouldn't be a bad idea as some people see the ones way up there and think they can't catch up.
The worry with that is, remove the carrot = losing players.
Its big risk to alienate a section of your community, even more so when traditionally its this sub sect of the community that subscribe the most money which in turn keeps the server up and running.
If you now capped everyone at 55, of which several people are slowly closing in on, you run the risk of them becoming bored and potentially quitting. The best way to curb it is to have higher increases in level size which over time naturally bunches people together.
Just for instance level 42-43 is 25,000,000
Level 43 - 44 is 44,000,000
This in itself causes a natural bottle neck slowing people down. If they have enough of these across the levels they can naturally limit progression while still allowing those who are "hardcore" or "power players" to keep pushing through. With the way Mir is designed those who play the most, get the most its always been that way. This is never going to be WoW where everyone gets to level 60 in two weeks and we sit round a campfire singing kum-by-ah with everyone having the exact same load out.