As it happens, I've just replaced a 17" Selespeed wheel on my wife's 156! So in the interests of informed debate, I've just nipped out an weighed it. 11.1kg (including valve and balance weights).
Here's a photo of my wife's 156 on them when we bought it.
I think they look nice (which is more than can be said for the wing!) but it's worth getting a tyre with good anti-kerbing protection! This car happens to be on 215/45 tyres instead of the 225s it's supposed to have. I think the ride is absolutely atrocious compared to her old 1.8 Twinspark, which was on skinny little ?195?- (can't remember now!) 60 profile tyres and steel wheels, but I don't know if Alfa did anything to the spring or damper rates on the JTS Turismo compared to the 1.8 Twink.
For what it's worth, I think any deterioration in ride quality will have a lot to do with the reduction in sidewall depth. You'll need to do that if you're going to keep the speedo accurate. The reason I've just replaced the wheel is becasue she hit a pothole and cracked the rim! There's just not enough rubber there to cope with our local roads (rural Cumbria) and it's not like she drives it hard! (Does anyone know if Series III Landrover wheels will fit a 156)?!
More seriously, I'd be very interested in swapping her current set of 17" Selespeeds with tyres for a complete set of 16" ones with tyres. Although the 17" ones look good, I reckon it's only a matter of time before she breaks another one!
(Oh, and while I'm on, if anyone has a blue (Alfa code 693) bootlid without that stupid "Max Power" spoiler on it, I'd happily swap that too)!