The Z has been praised by many car magazines for its rigidity. Many even said that the Z was originally designed as a convertible. I don't know how much truth is behind it. But a targa top is basically the T-top without the middle bar. Structurally, there shouldn't be much left to brace. As far as getting rid of the bar, any body shop can chop it off.
Going from a slicktop to a targa is a different story. But there isn't much more gained from a targa.