Well, I got a bit confused when I read this comment on Fireball in another guide:
This skill you should at least have 1 point in it. Maxing it is debatable, as you could easily get off 2 high damaging Fire Bolts equivalent to the time it takes to cast 1 Fire Ball with DoT damage. Unless of course, you are completely lucky with getting Fire Ball Mastery procs, then maxing it is more beneficial. Maxing this skill is completely up to you.
This sounds odd. Likewise at the first glance, the three basic skills (Colt Bolt, Fire Bolt, Lightning Bolt) sound unfair: "Cold bolt has 0.5sec casting time and Fire Bolt 1sec"....
I've haven't made an exact measurement here, but assume that beside the 'known' casting time, there is also a 1sec recast delay (or animation time). Therefore the total time between two Fire Bolts (the start of two consecutive casting time bars) equals 2sec.
Assuming this 1sec casting delay, the DPS (damage per second) calculation would look like this:
Cold Bolt: 38% damage / (0.5 + 1) sec = 25.3
Fire Bolt: 49% damage / (1.0 + 1) sec = 24.5
Lightning Bolt: 22% damage / (0 + 1) sec = 22.2
Now the damage output of all these three skills don't look that different anymore.
Regarding the initial quotation, The Fireball would also have a 1sec casting delay, which increases the total recast time to 3sec. Therefore, you can cast only 1.5 Fire Bolts (2sec total) by the time it takes to cast a Fireball. So let's also calculate the dps value of the fireball:
56% initial damage
8.4% after 2sec
8.4% after 4sec
Total: 72.8% damage
DPS: 24.3% (total damage / 3sec)
How did I get to this result? 56% is the initial damage, but the Fireball also does 30% damage of this initial damage distributed over the course of the next 5 sec. Basically, this means a short damage debuff that ticks twice with 15% of the initial damage. If you look at the resulting 24.3% dps value, it's quite similar to the Fire Bolt mentioned above.
So if my assumption of 1sec delay is correct, the competition between the three initial skills knows 3 winners:
Frost Bolt: additional slow effect for free
Fire Bolt: by far the lowest mana cost per second for the same overall damage
Lightning Bolt: highest mana cost, but highest mobility advantage and double DPS on frozen targets
But it also shows that there is actually no reason to cast a Fireball with the 'normal' casting time if you've maxed the Firebolt as well.
However, if I continue this calculation on to Fire Arms, things are getting interesting. Fire Arms grants a 30% chance that the Fire Bolt deals 40% of the initial damage over time and grants a 10% overall damage bonus. The calculation looks like:
Fire Bolt + Fire Arms:
53.9% initial damage (49% x 1.1)
10.78% after 2sec (40% of 53.9)
10.78% after 4sec
Total: 75.46% damage
DPS: 37.73% (total damage / 2sec)
So if the Fire Arms secondary effect triggers, it causes a boost from 49% => 75.46%.... a damage increase of 54%.
However, since this happens only with a 30% probability, the theorectical average damage increase is 23.3% (40% DoT / 30% + 10% overall). I also wrote the 'theoretical' increase, since DoT effects don't stack. That's why the best thing you can get out of it, is an always active trigger.
Let's postulate another assumption:
Haste rate doesn't just reduce the duration of the casting bar, but also reduces the above mentioned recast delay.
This means that all skills have a profit of haste rate. This means that even an assassin would attack faster while using a guardian, although none of his skills has such a casting bar.
If so, then the Wind Arms gets interesting here as it grants a 20% haste bonus. Although it's debatable as well if I could state this, but attacking 20% faster equals 20% damage over the course of time. Therefore the Wind Arms also grants (even without the triggered effect) a static 20% damage bonus, which is quite similar to the Fire Arms total bonus (especially if you accept certain loss due to overlap).
The conclusion would be that you might consider the Wind Arms as a Magician as well. It might be a bit of a tradeoff though. Fire Flower won't benefit from the haste rate, but you are faster at gathering Pyromaniac stacks and you profit from a reduced Frost Driver casting time as well. If you combine this with the initial comparison between Fire Bolt and Lightning Bolt (nearly the same damage output), you might even consider to prefer Lightning Bolt over Fire Bolt as well. (This means no Pyromaniac stacks and higher SP cost, but more damage if combined with Frost Driver and Frost Nova) and still allows you to use Fireflower).