Completing all 3 optional objectives plus the Prestige Objective grants a +2,500 XP bonus
The game often registers the XP gained for your account level, but since you didn't "finish" the mission, you can restart it from the safehouse and do it again for the same XP bonus.
Getting to Mastery Level 100 in Hitman Freelancer is a massive grind, especially since the XP requirements jump significantly after level 80
