What are your plans for the Farseer? Which dicipline will you be going for?
You'll want to infiltrate with your Scorpions to make use of their Shadowstrike rule, obviously.
A lone Autarch, I find, is a sitting duck. You'll want to have him start the game as part of another unit, to protect him.
That unit can't be Scorpions, or that particular unit will not be able to infiltrate, which means they will not benefit from Shadowstrike.
I'm also wondering if D-cannons are the right choice. With just 24" range, there's a limit to how much of the table they can reach. Even so, say they get to shoot - unless you're going to fire at MCs/GCs/AV, they are too expensive relative to the damage they dish out on infantry. Considering that Shadow Weavers are half as much for twice the range (though packing less of a punch, I'll grant you), it may be better to take two of those instead. That will free up 60 points.
Then there's the matter of the size of your Scorpion units; they are too large, I think. Consider taking 3 5 man units instead for a total of 15 models. A 5 man unit will still do enough damage to bring down most enemy units, while presenting less of a target. That's another 170p you can spend elsewhere, for a total of 230p. You can get a lot for that. A Dire Avenger Shrine is 195 points at its cheapest, and that's 15 additional Shuriken catapults with 18" range to support your Scorpions as you make your way toward the target. It's also three extra units you can use to shoot at different targets with, and three more targets in your army for your opponent to consider. It may not seem like much, but I feel like saturation is the key to a good list.
If you want a good place for your Autarch, get him a Shining Spears unit. Otherwise I feel like the Autarch is out of place. You can exchange one Scorpion unit for a unit of Spears (so that they are part of the +1 WS formation) using the remaining 35p after you get a DA Shrine.
Outflanking with a unit of Shining Spears is great fun. Just make sure you don't expose them to more enemy fire than they can handle. Especially with the Autarch riding along, they may present a choice target for your opponent.
If you don't Outflank too far away from the rest of your footslogging army, and/or too deeply beyond enemy lines, thus isolating them further, they tend to perform very well. They are powerful enough (on the charge) to take out just about any target.
Giving EMLs to your Guardians is a solid choice, especially if, as you say, you plan to have them sit behind and hold objectives. EMLs are always the most expensive weapon choice for us, and since they are free with the Guardian Battlehost formation, it's a good choice.
Your list does lack firepower, however, and having 27 Shuriken catapults (not counting the three Guardians that operate the EMLs) sit at the back may not be advantageous. You may need to use their firepower to thin out the enemy line before your CC units can break through.
If you insist on keeping them behind, the DA Shrine becomes ever more necessary, I would say.