I believe that the consensus is that MM can be competitive with SV at any level of ArP however for this to be true you need a ranged weapon of ilvl 226+.
MM below ~350-400 ArP should use 3/3 Improved Arcane Shot and include arcane shot in rotation, over ~400 ArP arcane should be dropped in favour of steady shot. Steady shot itself will still hit for less than arcane but will add to the rolling DoT for piercing shots if it crits and thus is worth more damage overall.
There may be conflicting viewpoints regarding this but I have both read & experienced the above & found it to be more-or-less true for myself.
On a side-note, don't gem for ArP straight away, keep gemming Agi until your ArP from gear hits the ~600-800 mark (depending on how confident you are of upgrades in the near future)