I love your goal. It will -- and should -- be disruptive. Be sure you assess your group's capacity to change, and that the leadership has a commitment to make this happen, not just "see if it will work."
1. Line managers are biased towards current performance because that's what your culture rewards. If you want this to shift, you have to shift what their rewards are based on, then support them in being able to identify high potential.
2. I'm guessing that line managers do not have a large enough view of your company's roles, so before you train them in identifying high-potentials, you want to look at your entire org system. Line managers can certainly be part of the solution but they can not own it fully.
3. The leadership team has to own talent development. There are appropriate responsibilities at each level of the org. to support the identifying and development process, and the leadership team are the drivers of the initiative.
4. Given your goal to build leadership/SMEs in next 3-5 years, my questions would be:
a. who's responsible for this goal?
b. what in the current system needs to change to have the goal be met? (compensation structure? role clarity, etc)
c. what in the current culture (mind-set/sacred cows) needs to change?
5. I'd suggest assessing your current 300 people and creating a profile of talent. Performance Style & Ambitions is a great tool, and has a robust back end to dynamically look at strengths, trends of who's in the talent pool. I'd be happy to show you online -- please contact me.
Don't start this unless the leadership isn't just aligned on the goal, but is clear that they will have to change, to make it happen. They will have to model the goal of spotting/developing talent before it becomes "normal".