Neil, I would be very interested to hear whether extending the COL makes any difference to the performance for you.
My reason is that I have experimented and found that COL makes little or no difference in my BSA. Clearly home-loading for the .22 Hornet is well worth while and a big improvement on the factory rounds that are normally available, but I am not sure about COL.
I remember reading something about the unusual chamber construction of the .22 Hornet. Perhaps others with technical knowledge will be able to help or comment on that aspect ? I am wondering whether this is why I experienced little difference in performance by changing COL ?
My old BSA is similarly limited by the magazine however I also tried single loading rounds that were longer than the magazine would take, with little or no difference to performance. The BSA albeit 1950's manufacture, is in very good condition and shoots better than I can.
I now set my COL for just short of the maximum that the magazine will take, however there is no 'science' in choosing that COL as such.
Good luck.