All manufactured items are built to tolerances. Lenses are no different.
The parameters of each individual lens element all have tolerances,
as do the dimensions of all the aluminum components.
Front and rear lens cells are matched at the factory to achieve the desired functional specs,
but those are also subject to tolerances.
The cells are mounted at the factory, matched to the particular shutter, which also has tolerances.
Since you don't have access to a large selection of cells to match to the particular shutter,
I suggest it's highly unlikely that you'll improve on the performance of a factory-assembled lens.