The belt needs to be the length it is fir the CVT to function properly. Portals lower the overall gear ratio significantly, but are a pretty expensive option. Not aware of anyone making lower ratio ring and pinion gear sets for the rear gear case.
A longer belt will not change the gearing, and would suspect it would just slip and fail in a short period. You can have the actual clutch sheaves machined to get a little lower gearing. JBS offers those services.