The X201 mods are pretty comprehensive, the 51NB team and others like XY Tech have commissioned entire replacement motherboards that carry i7-8550u or i7-10710u processors. See [1].
And the old 35W processors in some of these laptops will still deliver solid performance. The new ones will turbo higher, but throttle quicker. You get higher default clock speed and better thermals with the older processors. Of course memory bandwidth and peripheral connectivity are better with newer processors, but they're perfectly usable. If what you want to do to be productive is edit text and maybe render HTML and browse files, and your OS/IDE/browser does not place unnecessary demands on your computer, then an X201, especially modded, cannot be the blame for your lack of productivity.
And the old 35W processors in some of these laptops will still deliver solid performance. The new ones will turbo higher, but throttle quicker. You get higher default clock speed and better thermals with the older processors. Of course memory bandwidth and peripheral connectivity are better with newer processors, but they're perfectly usable. If what you want to do to be productive is edit text and maybe render HTML and browse files, and your OS/IDE/browser does not place unnecessary demands on your computer, then an X201, especially modded, cannot be the blame for your lack of productivity.
https://www.xyte.ch/