My experience has been 2 thumbs up for the strut brace on the CNV.
The way I see it, full body cars are more or less 'complete' as a structural unit, think of an aluminum can on it side. Realistically stout to torsional loads, (think grab both ends and twist).
Take that same can, shave a 3 inch wide section the length of the can off, now twist. No matter how much bracing to the lower portion of the car, (like the CNV bracing) the further from the base, the more likely to have some movement. Not enough to measure, create gaps and cause notable issues, but enough shift that the convertible feels 'lazy' in a corner. The brace helps tie everything together and makes for a better handling car. My thoughts are that like most manufacturers Pontiac had a chassis and had a great idea, 'lets make a convertible!', problem is, the chassis was not designed that way, and there are a number of changes to the vehicle to make it work, some better than others. Strengthening the firewall and crosscar beam would have reduced front end flex, but the car was already heavy.
I agree with OP, that on a closed body car, it's more eyecandy. On the convertibles, it is an addition that really does have a notable payback.
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