Fiat is european, not spanish. I believe they are french or italian. And Chrysler never bought Toyota, they are two totally independent companies.
And which is faster? a convertible or a sports car? That is a trick question, as many sports cars are convertibles. But, if you compare say, a hard top ford mustang to a convertible ford mustang of the same year, the hardtop is going to be faster. its lighter. a convertible top is heavy including the fabric, framing, and the motor to raise and lower the top. In addition, they also have extra structural support since they don't have a real roof.
And as far as the NUMI plant in Cali, GM and Toyota shared platforms on vehicles such as the Cavalier, Pontiac Vibe/Toyota Matrix and I think even the Chevy Aveo/Toyota Echo. While the two companies, GM and Toyota, remain completely seperate entities, they partnered together to share platforms on those few vehicles to reduct production and engineering costs.