RE: About O.S #8 plugs!
I'd have to agree with much of what you posted. Now flame propagation is a concern. This is why the fit of the glow plug is important. It is why one should not use an idle bar glow plug in a schnuerle ported engine, well only as a last ditch attempt. (The power a schnuerle ported engine has over a baffle can be traced to the smooth combustion chamber.)
In your case running a 5K feet of altitude it would be a benefit to experiment with chamber volume and glow plug heat ranges. I for one don't see anything magical in using the OEM's glow plug. For the most part these are just over priced. Now the Fox brand is usually price fairly and perform adequately for the sport pilot. I also don't change nitro content in my fuel. I find that making a new combustion chamber is cheaper and in fact often results in more power (the best of both worlds).