I did a video recently in which I tried to get 4 cups of water to boil on a single 0.5 oz Esbit cube. I used 4 cups of cold water straight out of the tap in my 1.3L Evernew UL cook pot and the Gram Cracker stove with my Sidewinder Caldera Cone system. I took it all outside to mimic actual conditions.
In the end, the water did not come to a boil, but it was steaming and there was a few tiny bubble formed along the bottom and edges. If the water were filtered before hand, this would have been hot enough for coffee, oatmeal, or whatever. But, in the end, I would plan up to 2 of the cubes for a full boil with 4 cups of cold mountain spring water just to be safe.
As far as soot, I have found that with the Sidewinder system I seem to get more soot with Esbit than I do when using my smaller Esbit set-up in which I use a Heine pot. Not sure why this is... then again, maybe the extra soot is a mixture of wood burning soot and Esbit... I dunno...
Anyway, here is the video if anyone is interested: