Wind, bike position, your frontal area, what you are wearing - #1
Tires - #2

Your motor is your motor - that won't change regardless of equipment.

All being equal - at 200W - rider position/tight fitting clothes and great tires vs crappy tires - you may see 1.5-2mph difference. Maybe 3mph if you do everything just right.

This speed increase will be noticeable without computers or timing. You can feel the easily difference.

IMHO - everything else falls into marginal gains. Things you probably won't really notice without a computer showing your speed. 18mph vs 18.2mph. 10 min up a climb vs 9:45 up a climb. You would really need to be sensitive to everything to notice those small gains.

The big gains - relatively cheap.
The marginal gains - $$$$$. And typically once you get to a certain point, the more you spend only equals smaller gains.

And a wicked headwind throws most of that out of the window. Best equipment in the world won't make much of a mental difference if you are pushing 250-300w to go 12mph.