From chassis side the Spyder is almost identical to the Boxster with sport suspension.
It's only the engine (and the front bumber) that is shared with the GT4. Weight savings are marginal when the missing comfort option like PCM, AC, ... are ordered. Only a naked Spyder will save weight. The roof is only 10gk lighter than the normal roof. All ather weight saving come from leavin out standard options.
I don't know why people think the Spyder will drive a lot different than an Boxster. It's the engine which makes the difference to a normal Boxster (GTS).
It will be a great car to drive top down, but no GT4 from driving dynamics and feeling.

The roof is not isolated like the Boxster roof. It is only one layer. You see the hole roof mechanics from inside and noises from outside are unfiltered. So even with top closed it is a loud car (road noise, ....). Absolutly no daily driver.