I tested on the enemy dummy and in an echo by applying 1 stack of spreading flames, checking the damage with Firestarter’s Torch and then with a wand with comparable increased dot damage, and the damage was the same. Repeated test by checking ignite damage with both weapons and also with and without spreading flames.
At no point was there more than just minor variance per application, and all had the same peak variance, there’s no observable damage bonus at all