Try checking the mag drop after a flight when everything is warm and after you've been flying for a while at cruise setting with the mixture leaned.
If it is better than just after a start then try drying everything and/or changing the spark plugs.
If there is no difference then I would still try changing the spark plugs before paying lots of money to have the mags overhauled. A friend of mine had an engine that kept stopping when the throttle was closed. It all got better when he changed plugs.