I still don't see the point. If you are winning, you can white peace them for 5 years. There's no "reduced timer" to spend it on, there.
What you're really saying is "I'm winning really well, I want to take stuff AND I want to be able to fight them again soon, so I can take more stuff." I just don't see how that's balanced. That sort of thing is why the truce timers were increased to begin with.
What is the strategic value, otherwise, of spending 20% warscore to reduce truce timer so you can sooner fight ANOTHER war in order to take a 20% province? Why not just take it now? Now is more valuable than later.
And this still doesn't address that increasing OR decreasing truce timer has value. What if I totally crush a nation just for one highly valuable province, then demand that the truce be extended 15 extra years so they can't even try to reconquest it for ages?
It doesn't seem remotely balanced, seems quite exploitable, and I can't imagine it would work well in multiplayer, which pretty much guarantees it ain't gonna happen at P.I.