I have seen all sorts of position recommendations, ... Some engines have shown a 1% gain in bhp., ...
I first read about doing this a number of years ago, but it seems to me much more voodoo than it is science. As you say, all sorts of orientations of the electrodes are recommended as optimum, which is the first sign things may not be what they are believed to be.
And, no matter what, a purported gain of only 1% is
very hard to have any confidence in. It means you would have to believe the dyno data was reliable to the nearest 0.1 h.p. for a number of runs, as well as all aspects of the engine itself being reproducible at that level. That is, you would have to be confident in the data and the reproducibility of all of the conditions such that your Vincent produced 50.0 h.p. with one plug orientation, 50.0 again to check reproducibility, 50.5 with another orientation, 50.0 again when returned to the original orientation as a check, 50.3 at another, and 50.0 again.
Although modern dyno outputs seem a lot better than old ones, much of that is because the output has been so heavily smoothed. For the same reason the display of a modern digital oscilloscope looks so much better than that of an analog unit. But that's because much of the interesting (and often important!) variations have been digitally removed for your viewing pleasure. Data from engine dynos are intrinsically noisy, which is why 1% strikes me as voodoo.
Maybe I haven't looked hard enough, but I haven't found any dyno manufacturer who even claims 1% accuracy and/or repeatability for their engine dynos. Chassis dynos are more problematic because each run everything in the entire system, from engine, gears, through chain, through wheel bearings, through the internal mechanics of the dyno heat up.
In a world where people pay a lot extra for large diameter, gold plated audio cables for their speakers because they think they sound better, it's not at all surprising that companies sell washers for a lot of money on the premise of a 1% gain. On the other hand, it's not as if those big speaker wires or indexing washers do any harm (except to someone's wallet).
Having said the above, in the case of very high compression engines where there is danger of the piston kissing the spark plug electrode, indexing can be essential. But to avoid mechanical interference, not to gain 1%.