We are going to use LTC3114-1 to meet our below requirements.
Input voltage : 20V to 30V
Output voltage : 24V
Output current : 800mA
Will the part LTC3114-1 support our requirement with above 85% efficiency.
Please refer page 5 of the datasheet which shows 24V output efficiency curve and Max load current vs Vin graph. You should be able to have a load current of 0.8A for the given input range.
I would guide you to the stronger LTC3115 for your power requirement and input voltage range.
Look at the example on page 36 of the datasheet.