With sharp (uneased) curves, no straightaway longer than 10', and two trains sharing a common track occasionally, 25 is about as fast as I want to go.
It's your layout, so run them at whatever speed makes you happy. But for all you fast runners out there who have a plethora of affordable loco choices going all the way back to 1938, I challenge you: Try one of those video camera flatcars, or Lionel's new camera caboose. SEE what your layout looks like at 40 scale mph.
"30:1 or bust! It's the gear ratio, it always has been the gear ratio."