Kinetic energy and galaxy arm rotation rate
It is miniscule. General Relativity is not required to understand the dynamics of galaxies. Motions are non-relativistic, a few 100 km/s at most, so the kinetic energy of objects is always much smaller than their rest mass energy.
Another way of seeing this is to calculate $GM/Rc^2$, a ratio which tells you the relative size of errors you will get in ignoring GR in any calculation.
For our Galaxy, we might say there is about $M\sim 10^{11} M_{\odot}$ within $R\sim 15$ kpc, and the ratio is then $3\times 10^{-7}$, which indicates Newtonian gravity is fine for most purposes.