I would understand it, if the Windows drivers were superior in any way
Usually they are far more superior than drivers for Linux. In fact, ATI is an example of a good company which cares about alternatives OSes with a small marketshare.
Seriously, ReactOS is not ready (yet) for massive usage, and I doubt drivers would work right away. But as for mining, I think it could theoretically produce the best speed out there because it's "small and efficient" (an example to illustrate - we can have ReactOS boot up to desktop within a few seconds, when Windows takes minutes). Like, the best of Linux (custom optimizations, etc) and the best of Windows (native optimized drivers, better kernel).