How does this build differ from your leveling and early raiding builds?
During most of beta it was pretty difficult to gear up. I found Vitality Serum a particularly effective form of survivability insurance in my "early" raiding career.
Note that I still recommend taking this skill if you are not yet geared (i.e. wearing Centurion PvP armor in PvE and/or wearing Tionese and below -- anyone with gear that is mostly at or below a 126-128 rating will probably get more benefit out of the extra health from Vitality Serum). You don't want to be the guy in HM/NM Operations with only 14k health! At that point you become a liability for your healers (i.e. you are a free kill for some bosses).
I prefer to use Orbital Strike in PvE when possible, so that upgrade was a natural choice to support my stronger gear. Explosive Engineering also improves Explosive Probe, which is never a bad thing.
- This build drops Sniper Volley.
Although useful in its own right, I don't notice a large difference between having the skill and not having the skill. It's a preference thing, so don't feel bad if you go for something else. It's pretty common for me to take Sniper Volley one week and drop it in another week. I go back and forth on whether or not I want to have it around.
- This build keeps two in Cover Screen.
There are a surprising number of attacks in PvE that can be mitigated by Cover Screen. Surprising. Probably not as effective a PvP choice, but a pretty solid PvE choice.
Sure, I could have taken two in Pillbox Sniper -- but I like both the activation time in Sector Ranger and the built-in knockback from Heavy Shot. I do a lot of PvE raiding with some occasional PvP warzones, so this build serves both purposes for me. Another "personal choice" section where you might choose to lose Heavy Shot.
Why raid with a pure Marksmanship sniper?
Put simply: Marksmanship is my preferred playstyle. I have the most experience with this particular tree and I can dish out a lot of damage and manage my resources successfully without spending too much time thinking about my rotation or priorities.
I'm sure there are other viable possibilities. In fact, I've played most/all of them.
Without a combat log it's impossible to provide a truly objective measure of "what spec does the most DPS." Rest assured that your preferred playstyle is probably supported by some form or variant of Sniper gameplay.
Does this mean there aren't improvements that could be made to each tree? No. But, as of this writing, all three trees should be PvE-viable in some form or fashion. I've had success with all three pure specs in both HM and NM 16m Operations content.
As for hybrids: consider building them or using them after you've played a few of the pure specs. You'll have more success tweaking your individual build if you have experience with each of Sniper's three trees.
Note: Shortly after this was posted, I tried this additional variant and took a point out of accuracy in order to move it into Pillbox Sniper. The 1% reduction in accuracy doesn't have much impact on me with my level of gear. I've been enjoying it.