I was originally writing this response as a private message, but decided to post it in the thread instead since it may have value to others.

The answer to your question does not only have to do with ArP (although that is a factor). It does not take much ArP at all to make the switch worthwhile. How much depends on a lot of other factors due to cross-coupling of stat effects so it is not worthwhile to quantify a number.

In addition to ArP, what I find to be a another discriminating factor on whether to drop IAS and switch to the ArP rotation is your crit rate. Since 3.1 and the introduction of Piercing Shots, crit rate has become more important for MM hunters. If your crit rate is high enough (which yours is more than high enough at over 50% unbuffed), then it pays to drop ArS and use an extra SS since it procs PS and ArS doesn't.

Other benefits of this change include:
1) A slight mana savings since Master Marksman reduces the cost of SS but not ArS
2) If you move the points in IAS into IB, it will greatly increase your AiS damage as well as your AoE damage
3) Easier and more consistent rotation to implement since it is always CS->AiS->SSx4

Here is a comparison with your current spec and the priority rotation on the left and the updated spec and ArP rotation on the right:

ArS: 5109 / 4443
SS: 4768 / 4768
AiS: 7199 / 8050
DPS: 8070 / 8112

This is a 42 increase in DPS with the 337 ArP you currently have. The SS that replaces your old ArS does do about 340 less damage (510 if you average 1.5 ArS per old rotation) but your AiS does about 850 more damage more than making up for that difference. Plus, as your ArP/crit % increases, the difference will become even larger.

Of course, you will still need to use ArS on certain fights like Hodir and Vezax where magic damage is buffed. In addition, when moving and having no other instant cast shots available, you will want to use ArS too, but even in these movement fights that you are concerned about, the loss in damage in ArS by losing IAS is made up for by having those points in IB buffing your AiS instead (with assuming a single ArS per rotation).

Hey, well just for starters, yes changing Grim toll for Wrathstone is probably a good idea since the proc from Grim toll is very good for MM hunters. (runestone of course being better)
The glyphs are quite good, I use the same ones, you can choose to switch to Kill Shot though, your choice.
The bug of Steady Shot to my knowledge was that the extra damage from glyph worked even when the target didnt have serpent sting applied.

Just forgetting the incoming 3.22 nerf I'd suggest regreming ArP (for now at least) and try hit ~600 ArP (without the proc) then you can drop Arcane Shot form your rotationt or at the very least not use it on the steady fights (where not a lot of movement is required)

Focusd Aim imo should be taken if you are dropping Arcane Shot - where else do you plan to use the extra 2-3 points? its either ArS or Focused Aim.

Death's Virdict is one of the best trinkets, a great setup would be that and runestone (thorim trinket)

The set gloves are not terribly great, no. But their relative weakness is more than made up for by the bonus (which is rather good). I say keep the two pieces and when you get a third exchange the gloves.

SS glyph not working with MFD was fixed in a previous patch.

ArPen MM build is always going to use SrS and SS glyphs.

Use the spreadsheet to determine the best final glyph as this choice is influenced by many factors.
(Possibilities: Hawk, KS, AiS, and CS.)