Let's say you do go to Apple, they install iOS 10 and you're unhappy with the performance. The best they'll be able to do (and it's highly unlikely because it will not fix anything) is give you a replacement iPad, which comes with the newest software anyway.
At the Genius Bar, there is no such thing as downgrading and they will not offer it as an option. I can guarantee you they don't even have the resources to do this in the store. If you are looking to run iOS 8 or 9 on your device, you will have to do it from home.