No attack currently has more than 1833 base damage, so use Colossal.
You made a mistake in the calculation. .8 base > .65 base + 178.75 (you need to clear the parentheses first)
.15 base > 178.75 base > 1191.67
Storm Owl base 1575 avg:
Extraordinary vs 35% resist = 1260 Colossal vs 35% resist = 1202
A few storm and fire spells have more than that, though I agree with most attacks Colossal is better in this case. The higher the resist, the lower the threshold for base damage to make Extraordinary more effective with more spells.