If a team has people who are passionate about organization's mission and projects, it is possible that they deliver more than what they committed. Donot see this as an opportunity for exploitation - an opportunity to assign more work, but collaborate with them to provide more space and time for working on things that they like to do. It is very rare to have such people in teams and don't take it for granted.
Real conversation below. Don't be that manager
Employee: We delivered this product even though we had limited staffing and budget.
People were very passionate about the project. Having more staff and time
can really help us to sustain this.
Manager: Well done. I really appreciate your efforts.
How do you people manage do this?
Employee: Yeah, thanks. What I was saying is...
Manager: You people are role model for other teams.