I would be surprised if there is significant interest in competing to finish campaign missions the fastest
For a speedrun community to develop, they usually play the same "version" of the game (usually the original because it's the buggiest, because bugs are fun for any%)
Every time a patch goes through, the prior speedruns would be ruined. The community would have to come up with a new speedrun path. That would be demoralizing. Updating the campaign missions would get complaints.
Why speedrun the campaign when you can speedrun loading a nuke on Dual Gap?
So I don't think it makes sense to use FAF resources to develop infrastructure to support speedruns as a separate game mode. If you could show significant user interest in speedruns (e.g. by having contests based on leaderboard placement) maybe people would be willing to commit FAF resources towards a more formal speedrun system. Also, if you could show significant user interest in speedruns, there would be 10 players who are "top 10" at speedruns and we could look to them to provide guidance about what FAF should do to encourage more speedruns. Their request for FAF resources, as actual "champions," would have more weight than your request.
If you establish a speedrun tournament (e.g., if you get the highest place on the leaderboard for an "Overmind" game that happens in Dec 2020 with no sim mods, you win an avatar, or you win cash) that would be the fastest way to find out whether there is community interest that would warrant more formal involvement from faf administration.
It's impossible to justify taking resources away from TMM to put them towards campaign speedruns, unless you could show that (and I think these resources would come from TMM, because making a parallel game mode is what TMM is, you would need the same coders to work on your project maybe)