[edit 25 Sept 2009: this was written before the last patch, and I have a new version that covers it after the update]
I’m sure you’ve read that all the cool hunters are gemming for armor pen, and seeing good results from it. The best possible DPS thread over at elitist jerks shows a very high setup using mostly armor penetration, however this information should be taken with a grain of salt. In fact, agility will almost always get you more DPS, and today we’re going to figure out why.
Agility adds DPS in a fairly steady manner. Every point will give you pretty close to the same DPS as the point before it. It has no effective cap, it increases the damage of all of your shots, and aside from hit rating, it’s the highest natural DPS per itemization point for most normal setups.
Armor penetration (ArP) adds DPS differently. It only affects our auto-shot, aimed shot, and steady shot damage. At high levels of ArP, it will eventually surpass agility, however once you’re in 25 man normal mode T8 level stuff, you’ll find that ArP is about as good as crit. To the right, you’ll see my DPS per attribute table from the online version of the DPS Spreadsheet. Now, at this point, switching all my gems from agility to ArP would cost me about 0.3 DPS per point. However, that ratio changes as I add more ArP- after switching gems and getting some ArP upgrades, I might only lose 0.2 DPS or 0.1 DPS compared to agility, until we reach the break even point. Bear in mind- getting to the break even point will cost you DPS, but will reward your itemization points more from there.
At least, that’s the theory.
So what’s the background info here? Well, the fantastic numbers that are at the end of the tunnel here come with conditions. First, you need to have a trinket that can proc a large amount of armor pen- specifically the mjolnir runestone or the grim toll. Secondly, when they proc, you need to fire as many auto-shots, aimed shots, and steady shots as possible. This means standing still.
The advice usually given is to go for ArP once you can reach a passive amount (not including your trinket procs) of 566 if you have the runestone, or 619 if you have the grim toll. This will ensure that you never go over the cap when your trinket procs. One of the problems of the older versions of the DPS spreadsheet is that it would average out the ArP gained from the proc over time, not calculating the fact that people would be over the cap when the trinkets procced. This has been fixed recently, and the DPS results for the ArP build dropped correspondingly.
Basically, going for an ArP build might increase your DPS on certain fights, but it’s very proc and RNG dependant. Also, on movement intense fights where you may not be able to stop and fire steady shots on a proc (most of Ulduar), you’d be better off with agility. Lastly, on any fights where you get a magic damage buff (general vezax, hodir), your build will not be optimized to take advantage, as ArP only affects physical shots.
If you are looking to min max fights where you can stand still, then by all means, use ArP. Just be aware that your DPS will suffer compared to agility hunters on many Ulduar encounters.