How does the diffusion coefficient of a species affect the limiting current for a RDE voltammagram?
I would recommend you take a look at the book Electrochemical Methods fundamentals and applications.

For the Koutecý-Levich equation (totally irreversible one-step, one-electron reaction):

$\Large \frac{1}{i}=\frac{1}{i_K}+\frac{1}{i_{l,c}}=\frac{1}{i_K}+\frac{1}{0.62nFAD^{2/3}\omega^{1/2}\nu^{-1/6}C^*_O} $

The higher diffusion coefficient is, the higher is the current.

The diffusion coefficient can be computed from the Koutecý-Levich equation if you know all other coefficients.