This mod tries to roughly equalize XP/time across all missions except Outlaw Raids (and Operations, for now). By default, Gold Rush seems to have the best XP/time rate while most other missions are by far inferior, the worst one being Kelly. For reference I merely rushed through all missions (solo, no stealth, normal) and noted the time it took to complete each one. Quite obviously the times to complete a mission varies depending on many factors, but this seemed like the best way to compare XP gains.

Gold Rush, Outlaw Raids (they're mostly for gold anyway) and Operations (didn't feel like recording times for them yet) remain completely unchanged, and while all other Raids have had their base XP buffed (considerably in some cases, less so in others), but Gold Rush should still be on top. This means that, in theory, you can't level up faster with this than you could in the umodified game, but instead you can roughly gain the same amount of XP in a given time no matter which mission you play.

On higher difficulties this might actually be a bit different considering that objectives are oftentimes extended or otherwise adjusted, leading to longer mission times and thereby less XP/time. I'll try to come up with a better, more fair solution which factors in more variables such as difficulty, but until then I still believe this is not the worst solution.