Regarding the trending of ArP --
I am sitting with about 250-300 ArP in gear alone. With gemming for it, I am getting near 450. I have the Grim Toll as well. According to this write up, ArP is the most useful stat for my gear level (5/5 Conq).
That being said, I am still seeing many feral dps running around stacking Agi over ArP (at the same gear level). From what I am seeing, in the same environment, with the same gear, they are coming out on the DPS meters much higher than me.
Rotations are spot on the same.
Am I missing something here?
As far as the current theory is concerned, once you reach the trinket soft cap on ArP (556), you want to start gemming agility (while maintaining the soft cap, of course). It's certainly possible to do well by simply gemming agility because you end up with a broader and consequently higher range of potential DPS than if you had gemmed ArP (Nightcrowlers work
is a worthwhile read, the section about DPS dispersion is about halfway down his OP). Also, there is a school of thought that values gemming agility on fights were burst DPS is important (like yogg).
Much of your DPS is determined by factors outside of your control (RNG, raid comp, boss strat). Simply because one person does well gemming agility does not mean that you will also do well in that setup--it could be that he's getting chained hysteria, his raid setup caters towards melee DPS instead of caster DPS, and he has a mangle bot (which you may not). His rotation might be 'looser', in which case he has a lower margin of error when it comes to CP generation and he needs the extra crit to maintain a correct rotation. He could have simply gotten lucky, had great RNG and consequently, great DPS. You should be choosing your gems based upon what your raid gives you in terms of composition and strategy--what works for you is more important than what works for someone else.
If you upload your combat logs to world of logs, you can look at a graph that shows you where buffs and debuffs such as rip, rake, and SR fell off, where you were clipping rip/rake, and the DPS dispersion that you experienced on a given fight--all would be useful for you to identify exactly what you can improve on in your gemming and rotation.