You can't only balance around a realistic performance in a generic setting. I'm not strictly looking at the maximum log, but upper quartile in general, where Picto is performing extremely well. I admit that the highest levels of play do not impact most of the playerbase -- but as part of the playerbase that exists at the highest level of play, should the job's balance for me not matter at all? I don't think it's really fair to say "your way of playing doesn't matter," even if my way of playing isn't popular. I would absolutely hate it if jobs with high skill variances got buffed even more to the point where they're all you see in Savage farms and party finders. I think you could argue that, since balance does not matter for the 90% as you say, they should not even be considered for balance since they aren't engaging at a high enough level for their takes to matter anyway. I don't subscribe to that argument personally, since I think every player's perception matters, and no level of play is meaningless, whether it's high or low.
At any rate, I'm talking about standard party finder, where a 50th percentile Pictomancer is doing more DMG than a 95th percentile RDM. The job is very strong regardless of how poorly people play it at all levels of play, so even the argument that the upper end should be ignored makes no sense here because even the lower end of Pictos are doing just fine for themselves. If we were to balance around that lower end, even median-and-upper-quartile ranges would skyrocket in strength. In general, I think you should balance around all levels, but the higher level tends to be the level where balance matters more.
As for the double caster argument, there are certainly fights where double caster is really hard, I don't dispute that. Fights should try and accommodate both double caster and double melee where possible. But I think when double melee isn't accommodated, it's because one melee simply can't play, whereas when double caster isn't accommodated, it's because both casters are having uptime struggles. At least the casters are getting to play their job, just in a difficult environment. But I can see how it's unsatisfying for primary caster players.
Also, as for not seeing much double caster at all right now, in the Savage community, it's pretty popular right now. Many teams run double melee because they think that's the standard, but PCT + RDM is really popular among prog comps right now (mine included! I'll be on RDM for the tier). It'l be interesting to see the world race.
And sorry the thread keeps going to Picto all the time! It's just really overtuned right now, so any discussion of double caster, casters as a role, etc. will eventually bring it up. I tried to swing back onto topic!