The possible short of it (as of update 22) is:
Loot caps at level 30 without a jewel of fortune (despite chest blessing).
Loot caps at level 31 with a +1 jewel of fortune or +1 guild buff?
Loot caps at level 32 with a +2 jewel of fortune (or +1 jewel with +1 guild buff).
Loot caps at level 33 with a +2 jewel and +1 guild buff?
I'm pretty sure the treasure map does not increase the max loot level from past experience.
Loot appears to cap at 5 levels above the character level regardless (max level 33 loot on a level 28 character)
This is my (incomplete) loot data that I've collected (the level of the chest and the corresponding plat or gold that you get):
22 259-279 plat or 2220-2380 gold
23 321-345 plat or 2506-2568 gold
24 371-402 plat or 2639-2808 gold
25 440-460 plat or 2848-2952 gold
26 500-520 plat or 3113-3204 gold
27 563-583 plat or 3330-3349 gold
28 622-649 plat or 3413-3516 gold
29 678-708 plat or 3605-3801 gold
30 741-767 plat or 3887-4012 gold
31 817-850 plat or 4017-4150 gold
32 893-928 plat or 4244-4400 gold
33 978-1003 plat or 4411-4611 gold
If people want to contribute missing plat or gold amounts I can fill those in as well.