You're asking the question backwards, it should be whether a Rinnegan user can improve their usage of it faster than Kurama can improve himself. Kurama is a gigantic battery that has witnessed who knows how many jutsu to learn. Kurama can knead chakra & use senjustu, the only steps after that are just to throw it out of his body and add shape manipulation or an element. Seems easy enough considering the guy can shoot Tailed Beast Bombs without hand signs despite it being harder to perform that Rasengan.