Doesn't change the image much, but the latency feels improved and the connection speed is much, MUCH better than it was with my launch PS5
Resolution won't be noticeable, but graphical settings, image quality, framerate will look better with a Pro though.
For example, there is a noticeable difference between 120hz turned on or off in Blops6 (even though Portal screen is 60hz) and FFVII Rebirth in Performance Smooth looks a tad bit more blurry than Fidelity.
Basically, everything apart from resolution that would be noticeable on your tv, will be noticeable on Portal.
And with yesterdays update, the difference in stutters and IQ is almost night and day.
Edit:
Just realized the difference in IQ in FFVII between Performance Smooth and Fidelity is because of resolution, so it's definitely noticeable.
Which makes sense, because RP is just projecting your TVs output onto Portal. As far as the console is concerned, it's rendering on whatever tv you got hooked up.