Is threadless a noticeable difference from threaded headset setups? I'm considering replacing my threaded aluminum forks, threaded headset, steel threaded stem and steel handlebars with threadless carbon forks, threadless headset and integrated carbon stem and handlebars; a large reduction in weight on my 700c road bike.

Aside from weight, what performance differences (if anything) should I notice or expect from the 1.0" threaded (guess) / 1 1/8" threadless headset change? Smoother turning? Or had the change in the industry been motivated on weight/production considerations?

One thing that I miss from the threaded setups is that the stem and forks can be engaged in a much stronger connection, such that it would take way more force to make them move independently. On my threadless setups, I can still force the handlebars to turn independent of the wheel, as the stem bolts strip out before they would provide as much clamping force as a threaded setup.
Threadless forks aren't a significant upgrade in any way other than weight and simplicity of setup/maintenance. 

Depending on exactly what the steerer tube is made of and how much is exposed above the headset, they can be stiffer when pulling on the bars in a sprint, but for the other 99.9% of the time there is no difference. 

Having said that, I'd would never go back. The advantages in weight and maintenance are well worth it. 

P.S. You should really double check your measurements, a 1.125 inch threaded headset is a relatively rare item. They were made, but only for a few years and as far as I know only on mountain bikes and tandems. It would be very rare to find one on a road bike of any vintage.