I have been contemplating buying a 25 WSSM for the past year or so, but have not been able to find brass in any decent quantity. I have reconsidered.
I believe you could expand the neck of .243 WSSM to 25 WSSM if you're careful and use plenty of lube inside the neck. Since this brass is so hard to find, and you want to limit any losses. you may want to buy a separate expander button.