I superelevated my curves by adding N scale foam roadbed under the outer rail. It is exactly 1/8" thick, so as Bob points out you get the right 6" of superelevation. (By the way, great photo Bob!) Turns out the slope of the N scale roadbed exactly matched that of the Homasote O scale roadbed I was using.
Just be sure you have easements going into your superelevation. You don't want an abrupt change in height. I used a series of cardboard shims