I don't really share that sentiment, Onno. Convertibles are always going to show less performance than their hard top counterparts, which sets a sort of precedent for a number of drivers who choose to buy the convertible compared to the Coupe.
I think most drivers with performance as a chief concern would gravitate towards the coupe version to begin with, so sacrificing a bit more weight shouldn't be too terrible of a price.
Besides, I think Ferrari has the gusto to pull it off. Take a look at their F430 Spider - its numbers are nearly identical to the F430 despite the extra weight of the strengthening beams in the base.