You mean 451 rim , so you run narrow high pressure tires I assume.. that will be most of the gain

over 406 wider tire 20"

then use road rim brakes then , as rim is larger Diameter. brake reach less..


i recc Aluminum rims

braking on carbon is not as effective, and needless overspending, unless rarely used pure race wheels

now Bi Fri has sources for 24 spoke hubs .. Shimano Caprio is made for small wheels a 9~26 t cassette.