First of all, an '06 Jetta is a MkV, not a MkIV. The two styles are significantly different.
The problem with fitting any VW seats other than those from a MkI or MkII is that the legs for the seats on later cars are a different width than the rails fitted to the earlier cars. Some people have had success heating them and bending them into place. Others have transplanted everything except the base from the new seats to the old seat bases. It takes a certain amount of creativity to make it work.
Keep in mind that from the factory, Jetta seats do not fold forward. Access to your back seat will be very limited if you choose to use those seats unless you figure out a way to make them fold.
Sparco seats will fit in a Cabriolet if you use the VW specific seat mounting brackets. I don't know if matching fabric is available, but my guess would be yes.