No because every time you "spread" Combustion, it overwrites the previous one and limits it to the duration of the new one. So spamming Inferno Blast after Combustion would be 100% pointless.
The goal of this set bonus, outside of helping you prepare a Combustion at the start of the fight when all your procs are up (if the set bonus procs), is to allow for mini burst phases. Based on it's 6 second duration, it will let you cast for example IB, IB, Pyro!, IB, IB *drops off* Pyro!. Not sure if there's any other point in it.
To do the math, we need to look at a lot of different possible scenarios to calculate the damage and I don't plan to do that, just laying down the ground work for whoever wants to try:
1. Fireball/Pyro (non crit, procs the set) + IB + IB + Pyro! (non Crit) + IB + IB + Pyro!
2. Fireball/Pyro (crit, procs the set) + IB + IB + Pyro! (Crit) + IB + IB + Pyro! (allows you to possibly continue the chain)
3. Fireball/Pyro (crit, procs the set) + IB + Pyro! (non crit) + IB + IB + Pyro!
4. Fireball/Pyro (crit, procs the set) + IB + Pyro! (non crit) + IB + IB + IB + Pyro! (allows you to possibly continue the chain)
5. Fireball/Pyro (crit, procs the set) + IB + Pyro! (crit) + IB + IB + Pyro! (also possible chain continuation)
Also there are combinations when after the Pyro! in the middle of the chain you cast only one IB and then another Pyro! and then if it crits you can go for another IB.
Is it possible that knowing you'll get a couple of Pyroblast! for sure is worth less dps than continuing with the Fireball rotation and possibly not get them?
While you guys may find the set bonus to be underwhelming, until I see some hard math, I'm pretty sure it's a nifty dps gain.