Is threadless a noticeable difference from threaded headset setups?
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.