Service Ceiling for a C85 powered 415-C?

I climbed to 9500 with a C85 with 48" pitch propeller flying over the Rockies enroute to Oshkosh. DA was around +2000 on a warm calm afternoon when I took off from Missoula heading East, loaded to 1400#. So that might be around 11000' on a standard day. IAS was down to 75 mph, so I wasn't going to get much more out of her. But it was enough to clear the continental divide by a couple thousand feet.

I'm hearing 13,000, Youtube saying 12,000, and others saying 11,000. Does anyone have actual numbers they've flown? If Old Man winter would be kind and give me a sunny day I plan on trying it in mine, meantime what are your experiences with max ceilings? This ties in to looking at long cross country flights with my wife come next spring/summer.