I agree 100% about the bearings.
I switched to oil impregnated brass bearings and found that it was hard to get good free motion in the x and y carridges.
The brass bearings have a much tighter tollerance than the linear "ball bearing" bushes and tend to stick, so it's vital to ensure they sit perfectly into the carridge.
Mine needed 1mm cases to make them fit since they were only 14mm and the linear bearings were 15mm... so I printed out cases.

The linear bearing mod only reduces sound AFIK, I don't think it makes prints any more accurate... that only comes from good belt tension and minimal slack in the drive system.

but for sure the metal gears and new belts really improved my circles.