If one trusts Hamibini and many do not, there is 2-3 watts to be gained per wheel from average bearings to top of the line Japanese bearings.
Before PBP, I tore my bike down completely. I replaced all of the bearings with low friction seal NTN bearings in the front hub and I also tore down the Powertap G3 hub and freehub. The Powertap used really mediocre bearings. I put in a properly run in new chain (three chains....on my recumbent) that was waxed. I put on new tires and new latex tubes. Cleaned and lubed the idler (pulley that is heavily loaded by the chain on my recumbent). I put on the rebuilt Rotor RS4X crankset.
Two stories related to friction.....
I went out and did a hard training ride some weeks before PBP with the fresh bike. I beat all of my personal bests and took a considerable amount of time off my 10 mile TT. Some of this was fitness, some of it was the honeymoon effect of cleaning up the bike, but some of it was lower friction. My power levels were considerably higher. I think one can feel 10 watts. I know I can measure it very easily.
On PBP, I started with some GI distress from something bad I had eaten. I know how much power I make at certain heart rates and I ride a a planned level and do not ride harder. At my planned power (measured at the rear wheel), it was taking my heart 140 beats per minute when it should have been 120 BPM. It was very perplexing. I also could not shift off the big ring. I did not figure it out until I got home. The bottom bracket is pretty complex on that crankset. It developed around 5-7 mm play. Based on HR, the extra friction was sucking up a lot of power. If I had pedal power meter in addition to the hub power meter, I could say for sure how much frictional losses were happening. But, I could clearly feel that something was wrong. I mean the diarrhea all the way to Brest might have also contributed. When I got home, I swapped out the crank and sure enough, the bike was faster.
I think it is really easy to save 10 watts at 16 mph just with tires and although this only earns you 1 km/h, it is free money in the bank. Most of the rolling friction tests are on drums and the relative ranking of the tires is accurate but they fail to give a real indication of actual frictional losses on real roads. I learned this lesson because I spent hundreds of hours doing aero testing on my bent using the chung method. It helps to also have the Crr measurements or good estimates. I was getting pretty high numbers for Crr using the best of the best racing tires. I asked Dr. Chung by email and confirmed that my measured Crr was pretty typical or actually pretty good for real roads. I am too cheap to throw 10 watts down the toilet and even 2-5 watts on an IGH is too much for this miser, especially if I have to live with 14% jumps in gear ratios......at that point, I would just go fixe. Apologies for the rambling