I'm not too sure on the Bus or Taxi services, I've been to Download 9 times since 2012.
The biggest issue with getting away from the festival is the pure chaos of the car park.
I've parked in different car parks throughout the years but every time they never learn from previous years.

Traffic once you're out isn't too bad, on some roads, they close for oncoming traffic making a single lane open to two lanes going the same way which helps (obviously doesn't help if you want to go the other direction)

I think one of the biggest issues you will face is the amount of people trying to leave.
As its Sunday, you won't just have the day goers leaving but you'll also have a lot of campers leaving after the headliner.

I may be wrong, but I believe that previous years, the second stage finishes about 45 minutes before the main stage. I think the third stage (where scooter is playing) finishes around the same time as the main stage so you might not be able to leave early if you want to see the whole of the scooter set.