first impression was those heads have been cleaned. They "look" like more usual sooty deposits have been removed.
However, i totally realise that yourself and Mark would not post to the effect you have to prove anything other than a genuine point, just in case that needs clarifying. Just my first impression. So what cleaned them....
A few q's ....
If injector cleaner is for injectors situated the other side of the valves how does it clean the combustion chamber. I realise it will be in the fuel flow obviously, but how does the same product clean injectors "and" heads. Its not claimed to clean the latter afaik going by it name? I also know nothing about the product itself, maybe it does claim to clean the combustion chamber, dont know.
That black film in the plenum. Any reason why that oil vapour from the breathers wouldn't burn in the combustion chamber to a degree specific to that individual engine?
Also is it possible to compare two heads from differant engines without knowing that that the stem seals are fine. Not likely all 6 would give the same sooty level in all combustion chambers but....
And i suppose to be thorough....
I am presuming sooty deposits are mainly due to oil in combustion chamber to various degrees...? At work, got distracted, lost train of thought. Dam!
Reguardless of cost for a minute. If injector cleaner works as suspected in this case, why pay for better fuel, or the other way, if fuel cleans why pay for injector cleaner? Suspect proving one or the other will take more time than we have here.
I am personally always suspicious of seat of the pants experiences relating to such small performance margins. Having lpg on my car, i quite often play guess the fuel. The switch is directly behind the stearing wheel on the right so its easy to align it so you cant see it. Then press the button a few times in Random order so as to loose track if lpg gs on or not. At low revs only,( due to a small hic up on gas at high revs), i have not been able to call it predictably, given i have a 50/50 choice. In short, put super in, ah yeah, must be better.....or is it just me.
Thats enough for now, posting before i get fuddled
again.

Edit.
Brackets added to last paragraph to make clearer.