….
also ordinary sheet rock has a 2 hr burn through and can be used as minimal insulation (r value is under 5) and as fire deterent but i ALWAYS used tile backer or durrock. ive done a dozen or more hoods for friends and myself.

 
Do you think the rest of your kitchen would still be there after 30 min of full fire?   … Fire rating (2 hr) is not a controlling factor. What you are dealing with in the wall situation is conductive & radiated heat in which wood or other combustible material can start smoldering inside the wall without you knowing it. If the wall surface is 170 – 200 deg F, then there is a problem if conductive & radiated heat is not stopped.   Most Codes require a heat shield & air gap, in front of the wall, if combustible materials are in the wall and clearance less than Code specified is provided.
 
The difference between Commercial cooking equipment & Residential cooking equipment (other than heat output) is that Residential equipment is design to be close to combustible walls of a residence.   Residential equipment is insulated; Commercial equipment is not and clearance  distance of commercial equipment from the wall to back of equipment is specified as 12″ (+/-) by the manufactures.  And, their warranties & liabilities are voided if the clearance is not provided.