Hey Dan,
If you keep that guitar in open C then yes, by all means go to a medium.
BUT, you better remember you've got mediums and not tune up to standard. Taylor 12s are designed and braced for lights tuned to standard; you can go medium gauge if tuned down a full step (I did this for a while on a 655 and it sounded great). So you'll likely be fine with mediums tuned down to open C, esp since your 12F is also the shorter scale. Try the elixir PBs ...I've used these on my 12er forever and it's a fabulous string.
Edward