This is just a general estimate based on something I vaguely remember from CB1. Also, CB2 is a new game, so using CB1 data to draw conclusions about spells in CB2 will most likely invalidate said conclusions from the start. But I'll do it anyway!
Assumptions:
- Fireball:Pre-Buff Cone of Cold damage in CB2 was equal to that in CB1.
- In CB1, Cone of Cold was superior to Fireball at 11 rounds.
The Math:
- Pre-buff, Cone of Cold did at least 37.5% more damage than Fireball:
11 rounds / 8 rounds = 1.375
- Post-buff, Cone of Cold does at least 92.5% more damage than Fireball:
1.375 * 1.4 = 1.925
Conclusion:
Cone of Cold does more damage than Fireball in fights lasting 7 rounds or more.
1.925(x-3) = x
x = 6.243