I still think I would try running a few tanks with some alcohol added first.

If you want to see how it is progressing, clean out the black soot from your exhaust tip and then keep an eye on what is coming out. I think I would take the slow approach first. You will probably not get black mushroom clouds anyway, but rather black damp soot on your driveway...so no fun is to be had in upsetting your neighbors!

The biggest risk in getting the carbon out without removing the head is that if you have very heavy carbon build up, the pieces of carbon can come off in pieces that are big enough and hard enough to damage a valve. Basically, what the carbon can look like is small chunks of coal. Now, how bad do you really think your car is? My guess is that it is not that bad if you have had it up to high speeds. However, it can still have quite a bit in there, and enough to make it worth, to be extra safe, taking a slow and steady approach first.

Good Luck!