Quote
but life might not be as long if it is operated in this position for a prolonged period of time.
You may find an effect that may impact lamp life in virtually any position, be it the melted salts pooling where are two seals with voltage across them with probe down, socket cement weakening by heat when base up, upper arctube wall exposed to higher heat when horizontal, etc.
Also some effects may not be shortening the lifetime per se, but leading to a more likely violent EOL.
Unless the given position is not allowed by the lamp rating, you do not know which one is the most limiting one, as each effect will be suppressed to some extend by the exact lamp design.
Operating the lamp in positions not formally allowed means also an unknown (to you) risk, you will never know what was are all the reasons behind.
And by the way I wouldn't even be surprised if even the manufacturer would not know how it will perform. The point is, when (assume a hypothetical example) some standard lamp format is specified for horizontal only, the manufacturer designing a product targeting that standard specs, will make sure it performs well when used horizontally as that particular standard spec mandates. But what would be his motivation to spend high $$$ in evaluating that design for vertical burning (reliability tests are the most expensive ones, as it needs a lot of pieces for long time test), when practically no one will be using it that way? So he simply won't have any data for that particular product vertical burning position...