My party came to Chult as 5th level adventurers (we began before the book was released; I used the Red Larch & environs part of PotA to amuse us while we waited) and levelled up to 16th (I think) before campaign end.
I used pretty much all the tricks there is to make the adventure work for characters that aren't just overleveled for the content, but much stronger than the baseline too (given "all options on": feats, mc, items).
In some cases (Ras Nsi's headquarters) I simply threw everything at the party at once. In some cases (King of feathers) they were separated and had to fight solo. In some cases I didn't change anything so they could wipe the floor with their opponents. In a few cases, I just pulled out the stops (Bag of Nails being given pretty much the most lethal sneak attack I think the game can yield; Acecerak using one of the best Time Stop PHB combos I could think of; an Ancient Green Shadow Dragon and its lizardmen tribe completely taking them by surprise) But mostly I upgraded the opposition - and since I'm lazy, by "upgraded" I mean replaced (a CR 5 devil became a CR 10 devil and so on...). I tried to limit the "just twice as many" approach, since 5E becomes boring and drawn-out if the foes' main resource is an absurd amount of hit points.
The big things was to forget about the death clock - even at 5th level, losing 1 hp a day is entirely unworkable (since it panics the players into ignoring everything that isn't a direct lead to Omu). The writer who included that ticking clock really must have been utterly clueless of what makes hex crawls work!
And that I denied the party any long rests during the hexcrawl (overland) portion (before Omu): that way even low-level fights had significance, meaning I could use the meaty random encounter info as written.
Apart from the hardcover my main sources was the Companion thingie (by that Power Score blog), and the Guild Adept content on Mezro.
Finally there's a thread around on the magic item shoppes I offered in Port Nyanzaru. Lots and lots and lots of useful items for any mid-level adventurer!