Great guide. Lots of useful information for snipers/gunslingers. Personally I prefer this spec for MM.
http://www.torhead.com/skill-calc#400rcrbbRsRGoZbfZh.2
Siege Bunker is great against all the smash monkeys, but outside of smash it is usually better to move out of the aoe than eat it. My playstyle tends to be more of a semi-mobile MM so this ability gives me limited usefulness.
It might seem odd for a single-target spec to go for more aoe damage, but Explosive Engineering also affects Explosive Probe. That greatly improves your burst and helps to generate yellow damage on heavily armored targets. This gives me up to 4k crits on Explosive Probes. The +10 energy gives me some buffer for the extra Explosive Probes and Orbital Strikes (putting these on top of yourself is a great melee defense).
The 3 second reduction on Leg Shot isn't worth 2 points in my opinion.
Diversion's usefulness is debatable outside dueling other snipers. My strategy is to attack until they toss diversion, run, then comeback and hit them hard when they switch targets. It would be great to have, but I tend to like the small buff to survivability from Cover Screen better.