It isn't a 40% damage buff. It is a 40% damage buff to EP, when played properly. EP is a majority of your damage, but not all of it. This means a 40% buff to a less than 100% of your damage makes the 40% less than 40% of your total damage. Factor in how annoying it can be, at times, to keep it going, and that number drops lower. Mythic Rhythm is for people truly trying to maximize their potential because they hit a brick wall. It takes density + near-perfect snap-shotting to be any benefit over Momentum or Determination.
I reccomend two paths, which have pro's and cons.
Path 1: Take mythic rhythm and master it. You will have slower progress as you master the passive but the upside is that you are setting yourself up for the finish line.
Path 2: Hold off on mythic rhythm until you hit a brick wall. Use determination or momentum until you can simply go no further. This could push you higher, faster, but ultimately will end up lacking that little extra DPS that Mythic Rhythm provides. If you take this route, you can gem higher solo; you really should be using groups to push gem ranks.
IMHO, Path 1 really is the option. Focus on knowing when to move. Know when that busted EP will drop off so you can spread the buffed EP. Keep moving and creating density.
I am currently working on GR64 and only JUST got my ancient weapons. I hope to break onto the leader boards very soon.