I mean, with the diagnostic test, you're basically trying the LSAT for the first time or after only a cursory study of the test material, structure, etc. It's a tough test, especially under real time conditions, and a 151 is what some people achieve after several months of rigorous practice. For all intents and purposes, 150 and above may even get you into a few schools (maybe not UBC or UofT, but you never know). I think you should be very content with that. 20-25 hours a week is a LOT to dedicate to the test in my opinion, but if you do that, your goal of high 160s is a very real possibility.
Keep up the great work and best of luck!