The EFI should handle leaning out the engine for altitude, so nothing needs to be done to the engine. Propping for high altitude is what's needed, and my SWAG is a 17P 3-blade, but that's a guess, might need a 15P (or 19???). Going 4-blade will help get out of the hole, but you'll need to run 1" less pitch than a 3-blade prop for the same RPM, rule of thumb thing type thing.
You said Bravo3, so that would be dual props, and those props sets are not cheap... It may be best to visit your local prop shop and see what their recommendation is, and see if they will swap props if you're not satisfied with the performance of their recommendation. This may be the cheapest, quickest and and easiest way to get the correct set of props.
If it was a single prop boat, I'd buy a good condition used prop that's close, then see if you're under or over the WOT RPM, and adjust from there. I boat in Utah at times, lakes up to 9000' and have experimented with several props... I drop 4" of pitch going from 1200' to 9000', and the boat performs marginally well, at least gets on plane, but this is with only 140HP.
Changing drive gear ratio is an option too, as mentioned, but an expensive option, and only something I would do to a boat that is going to live at that altitude for the rest of it's life.