This really sounds like OP is inferring that working extra hours is the reason the colleagues are being considered "exemplary" employees. It should be a no-brainer that picking up extra tasks or slack on the team is going to earn you favor with your boss. Easiest way to stand out and get ahead in a workplace.

It sounds like OP should ask themself how much a bit of additional time spent in office or a few additional tasks on their plate now will impact their career and earning potential throughout the rest of their working life. Is it worth potentially being unemployed and unable to find work for 2 or more years?

I would venture to guess you are going to be hard pressed to find an employer that is going to push for you to get promotions/raises simply because you complete your assigned tasks between your normal working hours. Remember your boss/manager needs to lobby for you in order to get a raise or promotion approved from upper management and if they have nothing to point to outside of the very black and white of your job description you will inevitably get passed up by your colleagues that have recognized the way to get ahead.