Top-level market for a Metsuke isn't such a great idea. The marginal tax increase might not even off-set the amount lost by investing in the buildings in the first place, plus the Chi research issue, as you mention.

You can get +2 xp Metsuke by building a Market in a province with Philosophical Tradition, too. Though I suppose Philosophical Tradition+top level Market upgrade would mean +4 xp Metsuke, so I guess it's the same question either way - how much money per turn does a good metsuke in a good province get you? Not just % tax difference, but real, effective difference. I don't have any clue how administrative costs work, but those seem like they decrease the actual value of a good metsuke by quite a bit.

What I would really love to know more about is how tax rate affects growth. The growth malus from taxes seems to change a lot. It might be that having a good metsuke is overall not that valuable because the increased taxes slow growth more. In other words, taxing 150 at 30% is really not better than taxing it at 25%, if that 150 is going to grow 1 wealth less under 30%.