Glyph of Combustion doubles the duration and cooldown of Combustion (as well as doubles that piddly weak direct damage portion), but nowhere does it say it actually modifies the total damage of Combustion.
What I found through tooltips (and the general ticks of Combustion) was that while my duration was doubled to 20, each tick's damage was halved (so basically the same damage, just now spread out among 20 ticks (before haste) instead of 10 ticks).
It's... not supposed to do this, right?
Example: I had a *20k DP2S Ignite (10k DPS) and a *10k DP3S Pyroblast DoT (3.33k DPS). Rather than doing 13.33k DPS as what should be intended, Combustion was doing half of that (or 6.66k DPS).
* These numbers are made up.