On the 308 which was tube framed there was not much of a stiffness loss going to the targa, it think it was like 10%. On the 355 which was a unibody cutting off the roof lead to a significant loss of stiffness so there was probably not much point in staying with a traga design when there was little to loose in going the whole hog. The nice thing about a targa though is when you want you have an open car, and other times it is close to a closed roof car without all the weight of a roof mechanisn and there is rollover protection. Although this last point is coverd by the newer roll barr systems.
I don't kow where that 10% number comes from. I've owned both 308 GTS's and GTB's and can tell you that the rigidity difference is very noticeable in real life. BTW, 308's and 328's are not very stiff by today's standards even in GTB form. I owned a 348 for a few years and can say that it is significantly stiffer than the earlier cars. Dave