IMHO, Overwhelming Crit isn't worth the 4-feat cost on a purely ranged toon; OTOH, it's still not worth going purely ranged on a pure rgr vs monkcher. So to some extent it's a question of max effectiveness vs sticking with a flavor choice.
When it comes to Slaying Arrow, remember that it will proc up to 4 times during Manyshot and that the Vulnerable procs from Imp Shock Arrows will apply to its dmg. It is also multiplied by crits, so say hello to Adrenalized Slaying Arrows.

In terms of burst DPS, nothing else comes close for a rgr.
Since you're already LG, you may with to consider a pally splash: that gets you Div Grace, access to Div Might (with Bane of Undeath Twisted), and LoHs to power Light the Dark while in US. And as an elf, you can take the racial AA tree if you really want the AA capstone.
If you do take the Shadow DM, I definitely recommend taking Extend as well. The U19 overhaul moved Displacement to Gtr Shadow DM and you can only get 4 uses of Gtr DM from racial tree (+1 if you have
eChimera's Crown equipped). At lvl 28, each use of Displacement lasts 168 secs., so four uses is 672 secs. (11.2 mins); Extend would double that 22.4 mins.