I see no difference in performance between tins of JSB Exact based solely on the stated head size. I currently have tins said to be 4.51 and 4.53, if there is a difference in grouping it is masked by my marksmanship.

Better and more experienced shooters than I maintain head size is a very important variable. There is also widely reported data demonstrating I am not alone in realising the only accurate part of the quoted head size is the "4." bit. What I have seen, is particular batch numbers giving tighter groups.