You should apply an expression to the property you want to effect, not to time-remapping.
There is no way to set the value of property B from within an expression on property A.
An expression should always yield a value that is used instead of the current keyframed value of the property it is applied to.
if you want to know the value of the Threshold you would use a = effect("Dust & Scratches")("Threshold"); But you can only read that value, not set it.
So, if you want Threshold to always be 60 regardless of it's keyframes or time, simply apply this expression to it: