Will using mid grade gasoline in my 2003 Infiniti G35 hurt it's performance?

It depends on the timing map. If your manual recommends the highest octane fuel but says you can use a minimum of 87, then yes, timing will be retarded by the engine computer and you will loose power. However, if it is recommended to run 87, then you will not gain any power switching over to a higher octane fuel (unless you modify your timing - ie, advance the timing).