When exporting expression-driven mogrts, many times expressions continue to slow down the mogrt, even in precomps, even if their layer.transform.opacity is 0;

I would propose an expression for enabling and disabling a layer entirely. The expression would have to have it's own editor on the layer itself, returning true or false. It would in effect set the 'eye' icon status. This pseudo-property could become visible directly underneath the layer in the timeline by alt-cllcking the eye icon.

This expression would be the first one the expression engine looks at, and only if it is blank or true, will it evaluate any other expressions (incl. opacity etc.) for the layer or the precomp.

This way it would be simple to create mogrts with alternatives for the user, each precomped into it's own layer and not loose performance.

It would also enable a design, where potentially expensive computations can be put on expressions in eg. sliders on a null layer, and if they are not relevant to the user choices the null layer could be disabled entirely.