And again unless you were able to do a side by side test over a long period with same loads, same speeds, same weather conditions and same servicing interval then inspect microscopically the interfaces you could never find out how much if any difference it has in real world use.
You seem to be implying that such a side by side test would be impractical or difficult, or at least that is what I infer from what you have written. And yet arguably the bicycle offers the possibility of performing just such a test, since there are two pedals which will experience very similar (if not identical) loads and conditions, so it would 'simply' be a case of using a different grease in each pedal, and then comparing them after suitably testing use. That is probably the tricky part: riding in sufficiently testing conditions frequently/long enough for any significant difference between the greases' performance to reveal itself, especially since good quality pedals will have seals which will delay the ingress of the water, salt and mud which are needed to put the grease to the test.
I imagine that a comparison of different grease in front and rear hubs might not be quite so valid because of variations in the loads and possibly greater likelihood of contamination entering the rear hub, but it might still be worthwhile. Similarly there are two jockey wheels, which could presumably provide a good test of non semi-fluid greases, since unlike most other bicycle bearings they lack seals. The Chris King external bottom bracket, which can be purged with fresh grease with a dedicated tool, would also allow different greases to be compared (or equally comparison of regular purging of one side with fresh grease vs. no regreasing of the other).
Such testing is something I have been wondering about now for a while. It's the sort of thing that probably no commercial cycling magazine would ever do, despite the interest and possible importance of the subject. It's far easier to write a short puff piece on the benefits of ceramic bearings, than to scientifically determine the most appropriate lubricants and the optimum lubrication regime.
Very long posts whilst great for a small handful makes it less useful for the majority
Perversely I think the opposite is the case. For the small minority who regularly take apart their pedals, hubs etc. and clean and re-grease them (or who have fitted grease ports), I suspect that the frequency with which the old grease and any contaminants etc. in it are removed means that they might see far less benefit from the optimum grease for that application than the majority of lazy so and so's like me who are, to put it mildly, less than scrupulous in maintaining our bikes.
I think that for many of us our interest in this subject lies in better understanding how little maintenance we can get away with. If a one-off strip down of a hack bike and application of grease A in place of grease B used in the factory means that the bike will then last an extra two or three years before it needs new hubs/wheels/pedals etc., then that is useful to know. Similarly some of the reports I have seen of corrosion in hub gears on this forum and elsewhere on the internet indicate that the type and frequency of lubrication of hub gears can make a huge difference to their likely lifespan and trouble-free performance.
In a sense our interest in the subject is the same as the various industries for which the countless different types of grease were developed. Those businesses are concerned with identifying the optimum lubrication for their equipment: changing grease can potentially significantly prolong the lifespan of expensive equipment, as well as reduce costly planned and unplanned downtime. Consequently a lot of research and money have been devoted to improving knowledge and improving the products available. If cyclists can take advantage of that knowledge and those products, I'm all for it.