As far as damage to the engine is concerned, rest assured; nothing happens and nothing will happen! I.A. whether you use normal petrol, HOBC, or 2 mixed in any ratio.
Yes, pick, power, performance and response will definitely increase when you will only use High Octane only (or mixed with normal one), but it will come at a cost.
If you can afford, then go ahead and use High Octane only. It beats normal petrol in all aspects (other than price of-course).