I can't speak for decarbonising pistons this way, but in general a blowtorch can burn carbon away fairly quickly by actually burning the soot and leaving a clean surface behind. I.e. If you need to clean out things like exhaust meshes or other surfaces where soot has been baked on, this works great, provided the material can handle the heat.
It might be an idea to try out (say on an old piston), a focused flame on the top only, with some gentle wirebrush action. I think this would work out alright provided one doesn't go nuts and warp things. Never tried it but i think it would work.
I've generally wirebrushed the tops with a combo of razor and wet'n'dry by hand. +1 with not going over the edges.
I usually try and get them as clean as i can where they are acceptable for reassembly, not necessarily spotless like a new piston. They are only going to get carbon on them again in a few thousand miles.
Running too cold/rich/cold plugs in the combustion chamber generally causes excess levels of it.
As an aside, a friend of the family years ago used to pour diesel straight down the carb of my fathers ford V8 to de-coke it while it was running. The exhaust came out white until he stopped pouring, fumes everywhere. The car ran great afterwoods, must have cleaned up everything, valves, the whole lot.
This goes along Hondamans idea with using gas/petrol. So perhaps a piston soak in diesel would be good as well. I know its a great degreaser in general.
