Besides this I am not real sure where to put my points, some idea I have are:
Vortex Time Bomb
Hmm I think you must be using another mod with Synergies. Artillery, Vortex Time Bomb and Flame Bot doesn't exist in Synergies, but it's added by the mod Improved Engineers summon or sth like that if I remember right.
Anyway, the build you were citing higher is exactly the build i'm doing. But I'm maximizing Coup de Grace because it deals a HUGE amount of damage when monsters are stunned (and bosses can be stunned for a very short time if i remember right ; that ha sbeen added by synergies ; i'm not sure though). But anyway it's a great trash remover if you raise coup de grace.
Then I have to try to raise up Fusillade.. I've read some topics about it, some people seems to like Fusillade, some seems to hate it. I've to test it.
Still, you can raise Tremor : it's a very good debuff ! When you get surrounded, you cast it, and it allows you some more time to escape and regroup the monsters in a column, and then shoot them. Also, the chance to stun in tier 3 is great.. But the debuff to physical damage is nice too, especially if you're playing with berzerkers or anyone else doing physical damage.
I don't very like immobilization drone.. It's not powerful enough to slowdown enemies because it can't affect every enemies in the same time. So I'm not a great fan of it.
But Sledgebot is good to maximize : when it's 15/15, the cooldown is so decreased than you can just spam it anywhere it's available. And I've never seen my Sledgebot being destroyed yet by an attack, so it's a great meat shield. Very useful if you're soloing.
Another thing nice when you're a canoneer and you want to move fast is to invest one point in Onslaught, and very quickly switch your weapon using the hotkey and jump. What I do is that my first weapon is a cannon, my second is a sword + shield. With my cannon, I have Blast Cannon (and I will add fusillade later to test) as my primary and secondary skills. With my sword, I have Onslaught and Ember Hammer (useful to destroy shields). That way, when you have to run in the whole map and you're so lazy because you're so slow.. You just switch to Onlaught and you do it the Mario way.
Raising Bulwark can also be a good thing ; 20% (or 10% ? can't remember the exact value) Physical DR is always good to take. But note that as you're not melee, you're not likely to suffer physical hits. It's your choice then..
I think that's all i know about the cannon engineer
EDIT : maximize first your offensive skills. The harder you hit, the faster you will kill monsters and that way they will be less likely to be able to reach you before you kill them. But if you were doing a Melee Engineer, I would have told you the opposite.
On Hardcore you really have to always hit monsters in a distance. That's why Blast Cannon is very interesting because of its range, and his knockback, and his stun effect, and his proc with coup de grace, and the fact that it pierces monsters. If you want to use Fusillade, note that it begins to be powerful only at higher levels, and it's not very safe for hardcore because it's more or less closed range.