I rushed my mini to TH7...then maxed from there (Almost max Def TH9, with mid heros and DE labs left to do).
I would say max at TH7....which is sooooo easy to get gold and elixer using barch or all gobs. At TH8 I would max def and lab and BK....make sure you max BK as it will make the start of TH9 better since you'll want your AQ down the entire time until she's atleast 15.
Granted I did skip some of the Elixer labs at TH8 (Pekka, lightning)....You will have plenty of time at TH9 to upgrade those as there isn't that much Elixer items at TH9 compared to the Gold and DE upgrades/Labs.

