Don't be too hasty getting rid of a car you love, there's plenty of room for babies and seats in the back of a GT. In fact by the time you've got a forward facing baby seat (some are rear facing), you can just pop your seat forward and grab the baby from the seat very easily. The boot is plenty big enough to get a good pram in. We find there's not so much difference in load space between our Gt and 156 SW, unless we're taking 4 bikes on the car.
I'd hold out and see how you get on. It's a typical ' baby coming' reaction, must have a big car, Missus fretting that it's too small. At some point you'll probably consider a people carrier, but a GT will do ya and you'll still be keeping a bit of individuality.
After driving my V6 yesterday, the only other car I really want is a 156 GTA sw, just love the engine!