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Hands-down the best money I've spent on the FLHTK was the Corbin DualTour & backrest. Right behind that was the 10" Windvest, the WO575 bars, and the highway pegs.
I bought my UC used, and didn't need to make many mods. However, the very best one I made was trashing the 4 inch shield and buying the LRS Ultra 8 inch with flip. Probably doubled my mileage capabilities per day.