I'm not a number cruncher... But she is certainly my favorite class to play.
With the build I use I'd say she really kicks in around lvl 14. She is viable much earlier, but it's around 14 when I start consistently coming in 1st or 2nd on the end game lineup.
Thanks for the very detailed post! Judging from your build order, it looks like I Was Never Here is the final piece that is necessary functionally speaking. As such, I've put Alch down for 10, requiring Flank Attack, Shadow Strike w/ upgrades, and I Was Never Here. I figured the Stealth upgrade isn't necessary, but let me know if people disagree.
With a Lego, on top of Walking Fortress you want to have To The Death upgraded as well as Counterstrike, those are core skills in view. A Lego at level 10 is far from optimal, at least on Peri. It can be played with success, though you won't be facetanking Bosses until you get to a pretty high level.
That's always been my feeling. Legionnaire is actually a big part of why I made this post in the first place. I love the class, but haven't played him much lately because of the leveling process. I actually checked the skill trees and he needs to be 12 to get War Cry and Walking Fortress w/ upgrades, Bear Mauls the Wolves, and Flow of Battle. Continuing on, he would be 15 by the time he got To the Death w/ upgrade, or 16 if he goes for Counterstrike instead. Going for both would require him to be level 18. I'll give him a range of 12 - 18 for now.
And all my Assassin build, albeit not using that passive, are all Perilous optimized. (Note that I did not use the term viable lol)
lol. Careful, though, "optimized" is an even more dangerous word 
What's your preferred build, then? Can you outline the build order, core skills, and core level?
I adjusted Assassin a bit to get First Blood and Cull the Herd. For the majority of players who don't have top Assassin gear, I think those two make a huge difference. I remember desperately needing them before I was blessed.