i think everyone is missing the most crucial point regarding balance adjustments. you need to be thinking about whether or not the performance of the ability is within bounds of what other DGs have access to. Bite was nerfed because its mana efficiency was off the charts. so they raised its mana cost (alot) and lowered its damage (a little). now its back in bounds.
so compare Spit to other similar direct damage abilities, and while allowing for some range of variation of performance find a way to determine if its within bounds or not.
try something simple like a comparison to other abilities that deal only damage and have a similar cooldown.
cooldown: 7 sec, cast time: 1 sec, max range: 20, duration: instantaneous
efficiency (damage/mana): 1 / 1.38 / 1.85 / 1.94
total max single target DPS (damage/cooldown) : 42 / 75 / 121 / 150
aggregate potency rating (DPS*efficiency): 42 / 103.5 / 223.85 / 291
Venom Spit (http://demigod.wikia.com/wiki/Venom_Spit)
cooldown: 7 sec, cast time: 0.3 sec, max range: 15, duration: 10 seconds
total damage (initial + DoT): 500/800/1150/1500
efficiency(damage/mana): 1 / 1.23 / 1.43 / 1.58
total max single target DPS (damage/duration): 50 / 80 / 115 / 150
aggregate potency rating (DPS*efficiency): 50 / 98.4 / 164.45 / 237
its possible to have venom spit on more than one target at a time as its cooldown is shorter than its duration. this is a small advantage for Venom Spit but was not considered for this analysis because it is not a very common situation and is kinda outside of the context anyway. if you want to start comparing multi target damage, you'll find that Spit is probably the worst ability in the game for that, in terms of efficiency.
there are still minor differences between the two skills so my strictly numerical comparison doesn't capture everything.
...however I think the numbers say quite alot. Fireball is actually a noticeably better damage ability than Spit in alot of ways.
when i did this same comparison between Bite and its nearest similar ability, Oak's Penitence, i found that Bite was like 2 or 3 times as efficient as Penitence despite having similar effects. it was obviously out of line. in the case of Spit and Fireball its obvious that the abilities are close, clearly within acceptable range of variation, and that Fireball is actually slightly better.
my conclusion from this is that Spit should not be nerfed.