Sounds like you are talking about DJJ/WMX? Overloading there depends who you are working for, not all of them are too bad (at least it used to be like that). However if it is going to be local flight crew, the only thing you can do it to put YOUR on-site manager to monitor cargo presentation and loading.
There are up to 10% variations in flight times on this route, combined with 5t turboprop's marginal difference in fuel burn between different weights, I'm a bit doubt you looking into right area to be monitored.