I do not hold my thumb along the wrist but wrap it around the wrist as for the safety I guess I am just used to the M70 safety as per the factory design but I have noticed some shooters do not wrap the thumb around the stock and one fellow was using a Merkel .470NE! I read a while back that Col. Cooper advised to use the thumb straight style to access the safety and on heavy recoiling rifles it avoids getting wacked in face with your own thumb , I have tried this with my .470 double but prefer the wrap grip style but back to original question for snap shooting yes the safety on left side would be quicker but it depends on what you have gotten used to over the years.

Steve