So lets actually look at numbers and what they mean. Lets say the base damage of a spell is 200 damage before applying potency. The damage equation divides the potency by 100 then uses this a s multiplier in the damage equation. So a fire 2 split btw 2 mobs would be (200x(100/100)))= 200 x2(mob)= 400 damage. a fire 1 has a potency of 150 meaning it will do (200(150/100))x1= 1.5 damage or 300 damage. Since you can get about 3 fire II's in and 5 fire 1's at lvl 37 before having to enter umbra ice. that's 5x300= 1500 damage from fire 1. 400*3=1200 damage. Once you add a third mob into the equation the fire 2 rotation grows to 1600 damage which means its slightly ahead but negligible amount at that lvl. You still need about 4 mobs without firestarter for it to be a noticeable difference better.

Funny story I had the exact opposite happen. I had a lvl 37 blm who would only cast fire II saying that his spell speed was so fast that he could cast fire 2 so much faster then fire 1 that it was stronger. I tried talking to him about time spent in umbral ice, how spell speed works, how potency works, so did the 2 other party members and all he did the whole run was tell us we don't know what we are talking and we are stupid. we all reported him but we never kicked him we just tried to teach him.