Unless OMB became totally outrageous in their estimates, the errors in a rosy scenario are likely to be relatively small for the budget year, but then they grow mightily in future years. Rosy scenarios are, therefore, not suited for justifying large initial tax cuts or entitlement increases. Instead, they work better as a means for justifying large surplus reducing measures that have a small effect immediately, but are then phased in over time. If the bill built in a mechanism for cutting off phase-ins when future deficits were projected, I believe that the danger posed by rosy scenarios would be greatly reduced. CBO forecasts cannot stray far from that of a consensus of outside economists.
Administrations have more scope for playing with the numbers.
Although not a lawyer, I believe that the rule could be written in a constitutionally acceptable fashion. It is true that sequestration is based on OMB rather than CBO estimates, but since we are worried about rosy scenarios, the OMB estimates would not be constraining. When OMB turns out to be less optimistic than CBO, the difference is seldom very large and in those times the Congress would have to pay a lot of attention to where OMB was coming out, but that should not be difficult.