I suck at making estimates on my tasks. I recently estimated a task for 2 weaks but finished it in 12 hours...

Better this way than the other way around. 😉

Ha dude, I’m going through Q2 planning at the minute. My team has a capacity of 25 man weeks, products requirements (only top priority tasks) is 64 man weeks.

... I’m pretty sure you’ll be ok telling your manager you are finished early.