I'd like to just make an all-encompassing comment to sum up my thoughts on the matter.
EDIT: certain recent discoveries have made a section of this original post irrelevant. I'll simply keep what still apllies.
What some tend to forget is that it isn't really the TASer who has to "deal with it", it's the audience. As a TASer manipulating 100 optimal hits is the same as manipulating 10. But if an overly long boss fight
grinds the flow of a movie to a hault then it's only hurting the movie's entertainment in the long run.
The way I see it, if you don't keep the audience in mind, why are you submitting the run at all? (If you didn't care to some extent you'd simply keep the run for yourself). What I'm getting at is that there has to be a line between appeasing the "OMG itz on Death Mode!" crowd, and the people that want a fast-paced categorical decimation of a game that ultimately results in an impressive TAS. Yes, the game should be seen as a feat above what human hands can do (as per this site's motto) but that doesn't mean using, or demanding the use of any difficulty is always the answer. That is demanding either extreme.
If we're going to use the "it's more difficult unassisted" argument, then I can say that from the perspective of RTA running I can't think of a single instance in which I've heard or seen a runner say something to the effect of: "the bosses patterns are the same as the ones I'm used to, but the extra health makes it much harder!". Muscle memory is a wonderful thing. If a boss doesn't change it's patterns, speed or anything else, most veteran RTA runners don't seem to be upset. The difficulty tends to come from other factors (health management, less frequent ammo drops, more spawning enemies or even enemies dealing more damage - not just that the enemies
have more HP). I'm not speaking for all runners, but please show me an RTA runner that's a master at their game(s) who says otherwise.
In sum: I think a case-by-case decision situation with guidelines rather than a hard and fast list of rules is the best. If there are questions, then let the runner(s) justify it (this for using hard mode as far as I'm concerned). If the audience is more entertained by a certain difficulty then it should be obvious which one to pick. If they find something entertaining in both (and both offer enough variety and can be justified well and agreed upon) then let them coexist. I don't think an easy mode run should obsolete a hard mode run, though. It strikes me as being much the same as using Japanese text to improve on an English run. But if a
hard mode run that is undoubtedly better optimized, but is slower due in part to the difficulty (i.e. not slower due to optimization issues, obviously) then I think it should be able obsolete
a normal or easy mode run that isn't as well optimized, or doesn't meet the run goal as well.
I was going to pick 