Also, consider using a shimano dura-ace cable and housing set for both the brakes and shifters (or some other, higher quality shifter and housing). The housing is stiffer and seems to be lubricated better, with the result being less friction on the cables.
And use those shims before exploring replacing the whole shifter.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter