After 5 Imp Scorches, the 15% increase will only increase the fire spell's initial damage. This 15%-higher fireball will then crit for 150% of its current boosted value. Now, the ignite tick starts: It will apply 20% of this 150% of the 115%-damage fireball per tick for 2 ticks. However, the Imp Scorch talent does not benefit this Ignite tick again.
So, Imp Scorch does not benefit Ignite directly, but it is boosted because the initial spell was boosted.
For a clearer cut:
Fireball, in this example, would normally hit for exactly 1000 damage.
With 5 scorches up, the fireball will hit for 1150 damage.
Turns out, the spell was a crit. That fireball now has crit for 1725.
Ignite will then tick for 345 twice, for a total of 690 Ignite damage. The entire thing has done 2415 damage.
Compare this to without the 5 scorches.
Fireball would hit for 1000 damage.
Turns out, it crit instead. It does 1500 damage.
Ignite will tick for 300 damage twice, for a total of 600 Ignite damage. The entire thing has done 2100 damage. The Imp-Scorch one was exactly 15% higher than this unmodified one, even including the Ignite.