Trying to get more exact values without rounding up, would give the following values:
- Level 11: 66xp_____820
- Level 12: 133xp_____820
- Level 13: 200xp_____820
- Level 14: 416xp_____820
- Level 15: 1383xp_____820
- Level 16: 3650xp_____2000
- Level 17: 7000xp_____2000
- Level 18: 10350xp_____2000
- Level 19: 13850xp_____2000
- Level 20: 17500xp_____2000
So, assuming you had a stockpile of deeds, books and imperial deeds, so rounding off wasn’t a factor, the point at which upgrading is no longer advised would be raised to above level 14. Beyond that point, you are always worse off if you upgrade before 9.2
Disclaimer: It’s worth pointing out that for a player to determine the best course of action between level 11 and 14, they should look at their current totals and what their totals will be if they go ahead with upgrading, as the fact values are founded up means what would be advised generally might not be best for a specific player.