Have a read, ripped from the Hunter's forum ...
"Myths:
-Ranged Weapon Specialization scales as your damage increases! Lightning Reflexes doesn't!
Actually, they both scale, however, RWS is on a linear curve (y=mx+b), whereas LR is on an exponential curve (y=x^z). RWS is on a flat +5% addition, LR does more +%DPS the more Agility you aquire.
-Barrage adds so much DPS!
Barrage is +15% damage to a shot fired every 10 seconds (we won't count Volley, as its use is limited, and essentially neglibile), thus a 1.5% DPS increase.
-Improved Aspect of the Hawk does much more than anything in Survival!
We can compare IAotH with Surefooted as "intangible" increases. Neither is accessable in the opposing spec (5/31/15 vs. 0/21/30), and they both add about the same DPS. IAotH is about an extra autoshot a minute (assuming perfect distribution). Surefooted adds 3% to hit, which is something you don't notice much. It is safe to assume both add roughly the same amount of DPS over an extended period of time.
OK, on to the math. Considering both builds will have Hawk Eye, Lethal Shots, and Mortal Shots from Marksman, and both Slayings from Survival (talents that add directly to ranged DPS; the issue at hand), we can limit the comparisons to the different talents. Marksman has the aforementioned RWS and Barrage, as well as Trueshot Aura, Survival will have Killer Instinct and Lightning Reflexes.
As one can tell, skills that focus on +% of something truely take flight as you load up on the basis of the buff. Thus, a Lightning Reflexes build with a lot of agility will be better than a LR build with not as much agility; the same goes for RWS, as you need a lot of base damage to really see the talent in action.
On to the formulae. +Crit% is easily represented as +1.3% DPS, and I will use the standard 28 attack power also equalling +1.3% DPS (28 AP = 1% crit) throughout, though the actual number does vary, it is not enough to warrant a much more complicated model.
So, what we have is Barrage adding 1.5% DPS, RWS adding 5%, and Trueshot Aura adding 3.57% (100/28) on the Marksman side of the fence, thus giving us a static total of 10.07% added DPS. On Survival, we have Killer Instinct adding 3.9% DPS (1.3% per crit, +3% crit). On to Lightning Reflexes:
At 400 BASE agility (feasable in blues), LR will add 60 agility, which is 120 attack power and 1.15% crit, or 5.78% DPS, bringing our Survival total to 9.68% DPS.
At 450 base agility (epic geared), LR will add 67 agility, or 134 attack power and 1.29% crit or 6.47% DPS, bringing the total with KI to 10.37%. We have just overtaken the added DPS of the Marksmanship tree.
Now, if someone has agility out the yinyang and has a base agility of 630 (full Naxx to the nines), they will have 95 extra agility, or 190 attack power and 1.83% crit, bringing the AGI bonus DPS to 9.17, and with KI added, 13.07% DPS added, an additional 3% more than Marksman."