Cush is short for cushioned drive, the "differential" if you will on shaft drive bikes. They work fine unless you have a leaker, when they run out of oil they tend to lock up. My 51 BMW locked its drive up at 50 mph, shaft broke, drive unit shattered, rear of transmission explode, which tore the frame in half. Not much a concern with newer bikes, much better metallurgy.
Lisa's bike runs a couple ounces of white lithium marine grade grease along with the gear oil. Much smoother, but not great for seals. Since her's has leaked since day one it shouldn't be a problem.
LA, some here over the years have thought I'm kinda strange, my first motorcycle was a 1936 HD Knucklehead chopper with apes and a foot clutch handshift (stock). And I always build my bikes for max performance, I had a 45 cubic inch (750cc) flathead Indian (Scout) that could just touch mid-90s mph. My Scout Jr (30.50 cubic inches) could just touch 60 mph.
My Savage isn't THAT modded, high bars, cut down seat, taller shocks, thicker fork oil. The rest is just cosmetic (fender, gas tank, exhaust) or mild hop-up stuff (airbox mods, exhaust mods, rejetted, full tire pressure). Haven't touched the frame yet.